TAChart Runtime FAQ

From Lazarus wiki

This article answers frequently asked questions related to usage of TAChart at runtime.


How to add a series at runtime?

Just create the series, set its properties and call the chart method AddSeries

  TATypes, TASeries;

function AddLineSeries(AChart: TChart; ATitle: String): TChartSeries;
  Result := TLineSeries.Create(AChart.Owner);
  with TLineSeries(Result) do
    // Series title for the legend
    SeriesTitle := ATitle;
    // Show data point markers (red fill color, black border)
    ShowPoints := true;  
    Pointer.Brush.Color := clRed;
    Pointer.Pen.Color := clBlack;
    Pointer.Style := psCircle;
    // Show red line segments connecting the data points
    ShowLines := true;   
    LinePen.Style := psSolid;
    SeriesColor := clRed;
  // Add new series to the chart

How to iterate through the series of a chart?

The series are accessible via the array-like property Series of the chart. Note that this returns only the most fundamental series type, TBasicChartSeries, and a type-cast may be necessage before being able to access the series properties.

The following example re-colors all line series in red:

  i: Integer;
  for i := 0 to AChart.SeriesCount-1 do
    if AChart.Series[i] is TLineSeries then
      TLineSeries(AChart.Series[i]).SeriesColor := clRed;

If you prefer the more modern iterators you can use the syntax

  TASeries, TAEnumerators;
  ser: TCustomSeries;
  for ser in CustomSeries(AChart) do
    TLineSeries(ser).Series.Color := clRed;

How to delete/hide a series at runtime?

  • If you don't need the series any more just destroy it: series.Free.
  • If you want to keept it for other usage, for example to insert it into another chart, call Chart.DeleteSeries(series).
  • If you want to hide it, but keep it in the chart, set the property Active of the series to false.

Axes and axis transformations

Fixed axis limits

Normally the limits of the chart axes are determined such that the chart covers the available area as much as possible. But sometimes it is better to keep the axis limit frozen, independent of the data range.

This code uses the Extent of the chart to define a constant range for the y axis between 0 and 100. The first example manipulates the Extent consecutively, the second one simultaneously by using an intermediate variable. Note that the first method may crash the program in the third line if, for some reason, the new maximum is smaller than the currently active minimum. Note that the second code freezes also the x axis; you'll have to call Chart1.Extent.UseXMin := false and Chart1.Extent.UseXmax := false to release it again.

Chart1.Extent.YMax := 100;
Chart1.Extent.YMin := 0;
Chart1.Extent.UseYMax := true;
Chart1.Extent.UseYMin := true;


  ex: TDoubleRect;
  ex := Chart1.GetFullExtent;
  ex.b.y := 100;
  ex.a.y := 0;

How to add an axis at runtime?

A chart can contain several axes which are stored in its AxisList. Since AxisList inherits from TCollection you can use its Add method to create a new axis:

  TAChartAxis, TAChartUtils;

function AddAxis(AParentChart: TChart; ATitle: String; AAlignment: TChartAxisAlignment): TChartAxis;
  // Create a new axis
  Result := AParentChart.AxisList.Add;
  // Axis orientation: calLeft, calTop, calRight or calBottom
  Result.Alignment := AAlignment;   
  // Axis title
  Result.Title.Caption := ATitle;
  Result.Title.Visible := true;
  // Rotate title of a vertical axis
  case AAlignment of
    calLeft  : Result.Title.LabelFont.Orientation := +900;
    calRight : Result.Title.LabelFont.Orientation := -900;
  // Show axis line
  Result.AxisPen.Visible := true;  

If you want to assign the new axis to AxisIndexX or AxisIndexY of a series you can use the property Index of the newly created axis.

How to add an axis transformation?

TChartAxisTransformations is a container for several TAxisTransform instances. It must be assigned to the Transformations property of an axis in order to be operative. It is created at runtime like any other component:


procedure TForm1.CreateAxisTransformationOfAxis(AChart: TChart; AxisIndex: Integer);
  AChart.AxisList[AxisIndex].Transformations := TChartAxisTransformations.Create(Self);

However, a bare TChartAxisTransformations component is useless. It must contain at least one TAxisTransform object. In the following code, we will add a TAutoScaleAxisTransform which is needed for paned charts. Please note that, although the TAxisTransform objects are collected in the TChartAxisTransformations property List which derives from TFPList, new objects must not be added by calling List.Add. Instead, assign the Transformations property of the new TAxisTransform to the TChartAxisTransformations component:


procedure TForm1.CreateAutoScaleAxisTransformOfAxis(AChart: TChart; AxisIndex: Integer);
  T: TChartAxisTransformations;
  transf: TAxisTransform;
  // Create the TChartAxisTransformations component (or use the one dropped in the form at designtime)
  T := TChartAxisTransformations.Create(self);
  // and assign it to the Transformations property of the corresponding axis
  AChart.AxisList[AxisIndex].Transformations := T;

  // Create the AutoScaleAxisTransform object
  transf := TAutoscaleAxisTransform.Create(T);
  // and add it to the TChartAxisTransformations component
  transf.Transformations := T;

Logarithmic axis

Data varying over several orders of magnitude are usually plotted on a logarithmic axis. For achieving a logarithmic scale a ChartTransformations component containing a TLogarithmAxisTransform must be assigned to the corresponding axis. Some optimization must be done with the axis marks in order to get "nice" labels. Don't forget to set the index of the log axis to the AxisIndexX or AxisIndexY property of each series.

The following code shows how an axis can be toggled between logarithmic and linear scale:

  TAChartAxis, TAChartTransformations, TACustomSource;

procedure LogAxis(AChart: TChart; AxisIndex: Integer; Enable: Boolean);
  axis: TChartAxis;
  transf: TLogarithmAxisTransform;
  // Determine the axis to be processed
  axis := AChart.AxisList[AxisIndex];

  // If an AxisTransformations component has not yet been assigned to the axis, we must create one
  if axis.Transformations = nil then
    axis.Transformations := TChartAxisTransformations.Create(AChart.Owner);

  if axis.Transformations.List.Count = 0 then
    // Create the log transform...
    transf := TLogarithmAxisTransform.Create(Axis.Transformations);
    transf.Base := 10;
    transf.Transformations := Axis.Transformations;
  end else
    transf := (TAxisTransform(axis.Transformations.List[0]) as TLogarithmAxisTransform);

  // Enable the transformation for a log scale, or disable it for a linear scale
  transf.Enabled := Enable;

  // Find "nice" axis labels
  if Enable then begin
    axis.Intervals.Options := axis.Intervals.Options + [aipGraphCoords];
    // Depending on the size of the chart and the data extent you may have to
    // play with the following numbers to get "nice" log labels.
    axis.Intervals.MaxLength := 200;
    axis.Intervals.Tolerance := 100;
  end else begin
    // Adapt these numbers, too. These are the defaults of a linear scale.
    // Often the Intervals.MaxLength has to be increased to avoid overlapping labels.
    axis.Intervals.Options := axis.Intervals.Options - [aipGraphCoords];
    axis.Intervals.MaxLength := 50;
    axis.Intervals.Tolerance := 1;

In order to toggle between log and linear scale it must be known whether the axis currently is logarithmic or linear:

function IsLogAxis(AChart: TChart; AxisIndex: Integer): Boolean;
  axis: TChartAxis;
  transf: TAxisTransform;
  Result := false;
  if Assigned(AChart) and (AxisIndex < AChart.AxisList.Count) then
    axis := AChart.AxisList[AxisIndex];
    if Assigned(axis.Transformations) and (axis.Transformations.List.Count > 0) then
      transf := TAxisTransform(axis.Transformations.List[0]);
      Result := transf.Enabled and (transf is TLogarithmAxisTransform);

In total, these routines could be called in a button's OnClick event handler to switch the left axis (index 0) between log and linear:

procedure TForm1.Button1Click(Sender: TObject);
  LogAxis(Chart1, 0, not IsLogAxis(Chart1, 0));

How to find the pixel coordinates of the point of the y axis?

Essentially, there are three coordinate systems which are involved when drawing a chart on the screen:

  • The inner-most coordinates are the pixels on the screen, TAChart calls them image coordinates.
  • The outer-most coordinates are the values plotted on the axes; these are the units of the data values provided by the chart source. In TAChart speak these are called axis coordinates.
  • In between these two systems are the graph coordinates. This is an inivisible linear coordinate system layed over the chart. The connection between graph and image coordinates is made by the chart methods GraphToImage and ImageToGraph. The connection between graph and axis coordinates is defined by axis transformations. The axis transformations provide methods GraphToAxis and AxisToGraph. Normally, where there is no transformation, TAChart uses an identity transformation, i.e. assumes graph and axis coorindates to be equal. When the transformations object has been assigned to an axis you can also call the axis' GraphToAxis and AxisToGraph methods. And when a series has been assigned to an axis (by setting its AxisIndexX and/or AxisIndexY) you can also use the series' methods GraphToAxis and AxisToGraph.

Suppose you have a chart with two y axes, the right one displaying values between 0 and 100, and you want to determine the y pixel coordinates of some y axis value, say 27. Knowing the axis coordinates (27) you must determine the graph coordinate. Since the right axis is subject to an axis transformation you can call Chart.AxisList[2].GetTransform in order to dermine the TAChartTransformations applicable to this axis; this also takes care of the identity transform when no external transformations object is used. Then call the transformations method AxisToGraph to get the graph y value. Finally apply Chart.YGraphToImage to determine the pixel value:

  TAChartAxis, TATransformations;

function YAxisToPixels(AChart: TChart; AxisIndex: Integer; y: Double):  Integer;
  axis: TChartAxis;
  grY: Double;
  axis := AChart.AxisList[AxisIndex];
  grY := axis.GetTransform.AxisToGraph(Y);
  Result := AChart.YGraphToImage(grY);

procedure TForm1.Button1Click(Sender: TObject);
  ShowMessage(YAxisToPixels(Chart1, 1, 0.2).ToString);


What can I do to avoid compiler error "Identifier not found 'TDoublePoint'"?

TDoublePoint is declared in unit TAChartUtils, add it to the uses clause of the unit. It also contains the declaration of

  • TDoubleRect which is needed for chart extents.

Other declarations, often not found when working with run-time code, are

  • TSeriesMarksStyle for the Style of axis and series Marks --> in unit TAChartUtils
  • TSeriesPointerStyle for the series pointer styles, e.g. psCircle, psRectangle --> in unit TATypes
  • geometrical functions (creation of TDoublePoint and TDoubleRect, overloaded operators for TPoint and TDoublePoints) are in unit TAGeometry