TAChart documentation

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TAChart is a package for drawing graphs, charts and other diagrams. It is comparable in features, but not specifically compatible, with Delphi TeeChart package. One substantial difference is that some features (e.g. data sources and axis transformations) are implemented via separate components instead of just properties. This leads to increased flexibility and opportunity for code re-use, but at the cost of some additional API complexity.


Series are the central part of TAChart. Most of the series represent data taken from data sources in graphical ways, such as lines or bars.

Constant line series

This is the simplest series type, representing "infinite" vertical or horizontal line. It can be used as "central axis" in function graphs or as a draggable marker.

Also, by setting Active=true, Pen.Style=psClear and UseBounds=true and appropriate AxisIndexX, it becomes "axis extender", making sure that given Position will always by included in the axis range.

Basic series

Basic series are mos often used, and include line, bar and area series. All basic series can be "stacked" by using multi-valued source. Also, all basic series fully support rotation and 3-D drawing.

Line series

TLineSeries can be used to draw give set of points, optionally marking them with shapes and connecting with lines.

You can get "stepped" look by setting LineType property to ltStepXY or ltStepYX.

Fast lines

Some charting packages include special "fast line" series to quickly draw line series from extremely large datasets (10000+ points). Instead, TAChart contains optimized fast path inside standard line series code, achieving comparable drawing speed. Line series will draw very fast if all of the following are true:

  • There is no marks.
  • There is no pointers.
  • LineType is not ltFromOrigin.

Some operating systems/widgetsets may additionally require that LinePen.Style=psSolid and LinePen.Width=1.

Additional speedups will be available if Source.Sorted=true.

You can measure line speed drawing on your platform with the "line" demo.

Bar series

TBarSeries represents data as a set or bars, extending from ZeroLevel to data points.

You can control bar width with BarWidthPercent property. Note that the it is measured relative to the neighboring bars. If the X values are not equidistant, bars will have varying width. To prevent that, set BarWidthStyle=bwPercentMin.

You can draw multiple bar series side-by-side by using BarOffsetPercent property.

Area series

TAreaSeries represents data as a polygon extending from the data points to either ZeroLevel line or infinitely down (if UseZeroLevel=false).

You can get "stepped" look by setting ConnectType property.

Multi-value series

Multi-value series require multi-valued data source, and use additional Y values as extra parameters to draw complex shapes.

Bubble series

TBubbleSeries represent data as a circles of variable radius centered at data points. This series require source with YCount of at least 2, and use first additional Y value as radius.

Box-and-whiskers series

TBoxAndWhiskerSeries represents data as rectangles with a medium line and two T-like shape protruding in both directions. Although in statistics box-and whiskers plot is supposed to be based in specific data quartiles, TAChart does not enforce this, allowing user to draw arbitrary plots.

With some effort, box-and whiskers series may be used to represent other charts different in meaning, but similar in appearance, such as Gantt diagram or Candlestick chart.

This series require source with YCount of at least 5, and use Y values as follows:

Index Usage
0 Lower whisker
1 Lower box bound
2 Medium line
3 Upper box bound
4 Upper whisker

Open-high-low-close series

TOpenHighLowCloseSeries represent data as a vertical lines with two ticks, as described here.

It usually requires YCount of at least 4, and use Y values as follows:

Property Default Usage
YIndexLow 0 Lower point of line
YIndexOpen 1 Left-facing tick position
YIndexClose 2 Right-facing tick position
YIndexHigh 3 Upper point of line

Note that although Y values are supposed to be ordered ascending along the table above, the series does not enforce this and will draw any supplied data.

Radial series

Radial series ignore axis transformations. You can see examples of radial series in "radial" demo.

Pie series

TPieSeries draws pie charts.

For each data point, pie series interprets Y value as a relative size of the slice, and X value as a distance of splice from the center of the pie (only if Exploded property is true). Slice colors can be set in data items, or taken from the hard-coded list.

Pie radius can be either set manually by FixedRadius property, or calculated automatically so that the whole series, including all labels, exactly fits the parent chart.

There are several options for label positioning, controlled by MarkPositions property:

  • pmpAround -- marks are drawn outside the pie, on the continuation or radius vector for each slice
  • pmpInside -- marks are drawn inside each slice
  • pmpLeftRight -- marks are drawn directly to the left or to the right of slice

If RotateLabels is true, each label is additionally rotated so that (if LabelFont.Orientation=0) it is parallel to the radius vector of its slice.

Polar series

TPolarSeries represent data as points in polar coordinates.

The origin of the polar coordinate system is defined in graph coordinates by OriginX and OriginY properties.

For each data point, X value is interpreted as an angle in radians and Y value -- as a distance from the center.

User-drawn series

Provides OnDraw and OnGetBounds events to allow arbitrary custom drawing on the TChart. Note that using TChart.Canvas directly is highly discouraged an will often not as expected.

Functional series

Functional series are recommended way to draw functional plots as opposed to, for example, pre-calculating function data and using line series. They provide scale-independent controls of smoothness vs drawing speed.

You can see examples of functional series in "func" demo.

Function series

TFuncSeries represents a one-dimensional function defined by OnCalculate event as a line.

The function is calculated for each Step pixels of the image, so you can use this property to increase "smoothness" or drawing speed.

DomainExclusions property allows to exclude some intervals from the function domain. Function series correctly draws discontinuity points set by DomainExclusions. Currently, DomainExclusions can only be set at run-time.

B-spline series

TBSplineSeries draws B-spline of given Dergee using De Boor's algorithm.

Spline segments shorter then Step pixels are represented by straight lines.

Cubic spline series

TCubicSplineSeries draws cubic spline using standard Numlib package from the FPC.

The spline function is calculated for each Step pixels of the image, so you can use this property to increase "smoothness" or drawing speed.

Data source must contain at least 4 points and have strictly increasing X coordinate.

If there are too few points, and csoDrawFewPoints option is set, line will be drawn instead of spline using BadDataPen.

If X values are unordered and csoDrawUnorderedX option is set, spline will be drawn ignoring offending points using BadDataPen.

csoExtrapolateLeft and csoExtrapolateRight options enable natural extrapolation to the left and to the right correspondingly.

Color map series

TColorMapSeries represent 2-dimensional function defined by OnCalculate event as a field of pixels, with color depending on function value.

The field is drawn as a set of rectangles of size StepX by StepY pixels. The function is called once for each rectangle.

Color values are defined by ColorSource, which must be sorted. For each data point is interpreted as having X value correspond to Color value. If the actual value falls between color levels, it can be either linearly interpolated (if Interpolate=true) or rounded down to the nearest level.

When Legend.Multiplicity=lmPoint, color map series will display color levels in the legend.


Data can get into a chart from various sources. They are implemented as a set of components derived from TCustomChartSource.

To assign a source to a series, you can set the Source property. If the property is left unassigned, the series will use its own built-in list source. Methods like AddXY are delegated to the current series source. Note that the list source is the only editable source, so after you assign, for example, a random chart source to the series, a call to AddXY will raise an exception.

Each data item has the following fields: X, Y, YList, Color, Text.

Sorted sources

If it is known that X values of the source are ascending, some additional optimizations like binary search become possible. So all sources have IsSorted property which helps determine that.

Multi-valued sources

Sources can contain multiple Y values for each X value. These values are stored in the YList field of the source data item. The number of Y values is determined by the YCount property. Note that the first Y value is stored in Y field anyway, so YCount = 3 means that values are stored in Y, YList[0] and YList[1].

Additional values may be used by various series -- for example, stacked bars or bubble charts.

List source

TListChartSource is a basic chart source, storing chart data inside itself. As such, you can use Add and Delete function to change source data.

The source also has DataPoints property to allow setting data at design time. This property is a TStringList, with each line representing a data point. Line consists of X, Y, optional YList, Color and Text values separated by | (vertical bar) character. Note that DataPoints property is designed primarily for sample and demo code. It is very inefficient, and you should not use it to add data points from the code.

You can control X value sorting by setting the Sorted property. Note when Sorted is set to true, list source sorts the data and keeps it sorted after insertion of new points. If inserted points are not sorted, this may result in quadratic running time. To avoid this, you should so either set Sorted to true only after insertion, or pre-sort your data.

Random source

TRandomChartSource source generates random data in the given range and is intended mostly to use in demos. You can also use it as design-time replacement for you actual data source. This will let you see and change the look of your chart without having to run the application.

Each random source uses its own independent random number generator to guarantee stability of its values.

User-defined source

This source may be used if you already have your data in memory, but in a format different from the data items used in TAChart. Using user-defined source to access your data directly instead of first moving it all into a list source may (or may not) be beneficial for speed.

You can of course also use a user-defined source to generate, filter or modify data.

Note that if the Sorted property is set to true, it is the responsibility of the event handler to provide actually sorted data.

Database source

TDbChartSource takes data directly from a database. It is contained in a separate unit to avoid introducing a db-aware component dependency into every project using TAChart.

The following properties contain database field names for data item fields:

Property Access method
FieldX AsFloat
FieldY AsFloat
FieldColor AsInteger
FieldText AsString

If FieldX property is empty, RecNo is used instead.

To get multi-valued source, set FieldY property to a comma-separated list of field names. Note that YCount will be set automatically -- trying to set it by hand will raise an exception.

Calculated source

TCalculatedChartSource is the source used for manipulating data taken from the Origin source. This source performs transformations in the following order:

  • Y reordering -- Y values of multi-valued source can be duplicated, removed or exchanged according to ReorderYList property, which is a comma-separated list of original Y value indexes. Step skipped if ReorderYList is empty.
  • Accumulation -- replaces each item's Y values by some function (for example, sum or average) of the last AccumulationRange items. Step skipped if the AccumulationMethod property is camNone.
  • Percentage -- replace each Y value by the percentage of total of all Y values for that item. Useful for drawing "stacked percentage" bar and area charts. Step skipped if the Percentage property is false.

Efficiency notes

Primary data source API allows random access. Nevertheless, many sources, in particular random, database and calculated, may exhibit quadratic or worse behavior if actually accessed randomly. TAChart itself takes care to only use sequential access (although it may require several passes). Sources optimize sequential access by using internal state. User code should be careful not to reset this state during chart drawing from event handlers or custom series code.

A notable exception is the list source, which is guaranteed to provide fast random access. It may be used to cache slow sources with the help of CopyForm procedure.

Coordinates and axises

TAChart uses three coordinate systems:

  • Axis coordinates (known in some other applications as object coordinates) -- this is the "raw" coordinate values obtained from the data. As the name implies, axis coordinates are interpreted in terms of specific axis -- the same coordinate value may have different meaning depending on the axis it is applied to.
  • Graph coordinates (aka world coordinates) are converted from the axis coordinates using axis transformation, such as logarithmic scale. Graph coordinates are common for all objects in the chart.
  • Image coordinates (aka screen coordinates) are converted from graph coordinates based on the chart viewport. This transformation is always linear and can be influenced by chart tools such as zooming and panning.

You can add or remove an arbitrary number of axises by editing AxisList property. By default, chart have two axises: one horizontal and one vertical. They are accessible via BottomAxis and LeftAxis properties. Note that those properties are aliases to AxisList[0] and AxisList[1], so if you remove those default axises, accessing BottomAxis and LeftAxis will return nil.

Axis transformations

Axis transformations are grouped in the TChartAxisTransformations component. It contains a list of transformations which are applied in the order given. (For example, performing scale before and after logarithm will yield different results).

For transformations to have an effect, you should:

  • Make sure Enabled property is true for all transformations.
  • Assign transformations component to Transformations property of at least one axis.
  • Assign AxisIndexX and/or AxisIndexY properties of the series to the appropriate axis index.

Note that by default, AxisIndexX and AxisIndexY have a special value of -1, which means "ignore axis transformations". Also note that if you add or remove axises, the indexes may change. You can rotate the series by assigning both AxisIndexX to vertical axis and AxisIndexY to the horizontal axis.

Linear and logarithmic transformation

Those are simple arithmetic transformations.

Auto-scaling transformation

To display several independently scaled series, assign them to two or more axises and apply TAutoScaleAxisTransform to each axis. See "axistransf" demo, page "Linear", checkbox "Auto scale".

By using MinValue and MaxValue properties you can control the in graph coordinates of the auto-scaled series. For example, by setting one transformation to a range from 0 to 1, and another to a range from 1 to 2, you will confine all the series using the first transformation to the upper half of the chart, and all the series using the second transformation to the lower half (assuming there are no unassigned series left).

User-defined transformation

You can create you own transformation either by inheriting from TAxisTransform, or, if you prefer "visual" programming, by using TUserDefinedAxisTransform. In either case there are two basic requirements:

  • AxisToGraph and GraphToAxis functions should be defined everywhere in data range and inverse of each other (for example, avoid now only dividing, but also multiplying by zero).
  • functions should be monotonic.



Chart tools define reaction of the chart to various user actions, primarily mouse movements and clicks. You can see examples of tools usage in "tools" demo.

Tools are grouped in TChartToolset component, which should be assigned to chart's Toolset property. Same toolset can be used in several charts.

If Toolset is unassigned, for compatibility reasons built-in toolset consisting of drag zoom and [[#Reticule tool|reticule] tools is used.

Each user action, tools in the toolset are processed in order, and for each tool:

  • If Enabled=false, the tool is ignored.
  • If Shift is not equal to the current shift state, the tool is ignored.
  • Tools is asked to process the action.
  • If the tool signaled that the action is handles, processing is stopped, otherwise it continues to the next tool.

Extent tools

Extent tools modify chart's logical extent.

Zooming tools can be animated by setting AnimationSteps to the value greater then 1 and AnimationInterval (in milliseconds).

Panning tools can be restricted to the chart extent on all or some directions by using LimitToExtent property.

Zoom drag tool

TZoomDragTool allows user to zoom in by drawing rectangle with the mouse. The rectangle then becomes the new logical extent. Drawing rectangle "in reverse", from the bottom-right to the upper-left corner, restores zooming to the full extent.

RatioLimit property lets you restrict zooming to one of the coordinates, or require it to keep original proportions.

Zoom click tool

TZoomClickTool allows user to zoom in or out clicking on the chart with the mouse. ZoomFactor is the multiplier of scale applied by the tool, so factors below 1 represent zoom out, and factors above represent zoom in. ZoomRatio allows to set non-proportional zoom by indicating ratio of X to Y scale.

If FixedPoint is true, the location of the mouse click is used as a fixed point for zooming, otherwise chart image center is used instead.

Pan drag tool

TPanDragTool allows user to move logical extent by dragging mouse in the directions indicated by Directions property.

Pan click tool

TPanClickTool allows user to move logical extent by clicking inside Margins pixels from the corresponding border of chart image.

The panning offset is determined by the distance from the edge of the chart (the nearer to the edge, the greater). Setting Interval in milliseconds will allow to continue panning with the given interval until the moue button is up.

Data tools

Data tools are linked to specific data series.

Data point drag tool

Data point hint tool

Reticule tool

User defined tool