TAChart Tutorial: Function Series

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Revision as of 00:52, 26 September 2012 by Ask (talk | contribs) (Plotting y = tan(x): fix formula for tan discontinuity points)


Did you work through the Getting started tutorial? In that tutorial we had used some mathematical functions to demonstrate the basic usage of TAChart by means of line series. Line series, however, is not the best choice for drawing mathematical functions. TFuncSeries is much better suited to this purpose. This is a series type that -- at first sight -- looks like an ordinary line series. But in the background, it is completely different. It does not get its data from a ChartSource, but from a mathematical function. Whenever the series needs data it calls the handler for OnCalculate event where the user can pass the function values for an x value requested. This saves memory for storage of the function values. But most of all, it allows to calculate the function values, depending on zooming level and chart size, at sufficiently narrow intervals, such that the series curve is smooth even at high magnifications.


Let us start a new project with a standard TChart component on the main form. Modify its properties as you like. We will use the following settings for the rest of this tutorial:

  • Align: alClient
  • BackColor: clWhite
  • BottomAxis: Grid.Color = clSilver, Title.Caption = 'x', Title.Visible = true, Title.LabelFont.Style = fsBold
  • LeftAxis: Grid.Color = clSilver, Title.Caption = 'y', Title.Visible = true, Title.LabelFont.Style = fsBold

The resulting form is displayed in the following image on the left:


Adding a TFuncSeries

At first, let's draw a sine function, y = sin(x). We double-click on the chart, the series editor opens ("Edit series"). Click on "Add" and select "Function series" from the drop-down list. The chart will display a line going from the bottom-left to the top-right corner as a representative of the FuncSeries.

We could give the series a red color. Function series don't have a SeriesColor property, but we can use the property Pen for this purpose: Set Pen.Color to clRed.

Now we go to the page "Events" of the object inspector, and double-click on the event OnCalculate. This is the place where we define the function. OnCalculate is called whenever the series needs the y value for a given x value:

procedure TForm1.Chart1FuncSeries1Calculate(const AX: Double; out AY: Double);
  AY := sin(AX);


When we compile we'll see the sine funtion. But it is not the full wave - because we did not set up the axes, x runs only between -1 and +1, the default for an empty x axis extent.

Setting the extent

x extent

To get a wider axis range we have two options: we can set the extent of the series or the extent of the chart. There are subtle differences between both cases, in our simple chart, however, they won't show up, and therefore, we will not discuss them here. So go to the object inspector, select the series' property Extent, set the axis start at XMin to, say, -10, and the axis end at XMax to +10. Activate these axis limits by setting UseXMin and UseXMax to true. Now when you recompile, you'll see the sine function between -10 and 10.


y extent

Why don't we play with the function a bit to see what happens? Go to the OnCalculate event handler again and multiply the sine function by 2:

procedure TForm1.Chart1FuncSeries1Calculate(const AX: Double; out AY: Double);
  AY := 2*sin(AX);


Oh -- the function series does not automatically update the y extent! This is true for the release version of Lazarus v1.0. In the trunk version, however, the FuncSeries has a new property ExtentAutoY which, if set to true, enforces automatic calculation of the y extent. Please note that the property applies only if both Extent.UseXMin and Extent.UseXMax are true.

If you don't have the trunk version of Lazarus you have to set the extent manually: Select the series Extent again, set YMin to -5, YMax to 5, and UseYMin and UseYMax to true. This will show us the full sine curve with amplitude 2.



Now let's focus on the statement in the introduction that function series are always smooth, even at high magnification. To see the difference, we plot the sine function a second time, but now as a line series which draws the function only with predefined segments.

Add a "line series" to the chart, set its Pen.Style to psDash, and add code to the form's OnCreate event handler which defines the series data:

procedure TForm1.FormCreate(Sender: TObject);
  N = 100;
  XMIN = -10;
  XMAX = 10;
  i: Integer;
  x: Double;
  for i:=0 to N-1 do begin
    x := XMIN + (XMAX - XMIN) * i / (N-1);
    Chart1LineSeries1.AddXY(x, 2*sin(x));

When you compile you'll see an overlay of the red function series and the black line series, the curves are hard to separate. Now zoom into the chart: while holding the left mouse button down drag a small rectangle near one of the maxima of the sine function (you need to drag from the top-left to the bottom-right corner to get the zoom effect). After you release the mouse you'll see a blow-up of the zoomed rectangle. If the magnification is large enough, you will see the segments of the line series, but the function series will still be smooth.


Before we continue, delete the line series -- we don't need it any more. Remove also the FormCreate event handler.

Domain exclusions

Plotting y = tan(x)

In this exercise we want to plot a different function, y = tan(x). We can easily adapt our project to this function by changing the OnCalculate event handler:

procedure TForm1.Chart1FuncSeries1Calculate(const AX: Double; out AY: Double);
  AY := tan(AX);


The chart looks fine at first sight, but when you remember some basics of the tan function from school you'll notice that the (almost) vertical lines near +/-1.6, +/-4.7, +/-7.9 are not correct. The function is not defined at these locations (to be exact, at (n+1/2)π). The lines should not be there. Because the function series does not "know" about these discontinuities, it draws a connection line between the last and first calculated point before and after the discontinuities.

TFuncSeries provides so-called DomainExclusions to overcome this issue. These are points and regions at which the function is not calculated and not drawn. Presently, DomainExclusions do not appear in the Object Inspector, but must be assigned in code at runtime by calling their methods AddPoint or AddRange.

In case of the tan function, we add the following code to the form's OnCreate event handler in which we exclude above-mentioned points from the calculation:

procedure TForm1.FormCreate(Sender: TObject);
  with Chart1FuncSeries1.DomainExclusions do begin
    AddPoint(pi/2);      AddPoint(-pi/2);
    AddPoint(3*pi/2);    AddPoint(-3*pi/2);
    AddPoint(5*pi/2);    AddPoint(-5*pi/2);

This chart, now, is perfect.


Plotting y = ln(x)

In the last exercise, we add another function, y = ln(x). For this, double-click on the chart again, and in the series editor add another function series. Set its color to clBlue, and write the following OnCalculate event handler which tells the series to plot a log function:

procedure TForm1.Chart1FuncSeries2Calculate(const AX: Double; out AY: Double);
  AY := ln(AX);


But when we run the program it crashes because of a floating point exception! Where does that come from? Our x axis starts at -10, and the logarithmic function can be calculated only for positive x values. What can be done against that? The answer is domain exclusions, again. We just forbid calculation of the function for negative values and for x=0. For this purpose, modify the form's OnCreate event handler as follows:

procedure TForm1.FormCreate(Sender: TObject);
  with Chart1FuncSeries1.DomainExclusions do begin
    AddPoint(pi/2);      AddPoint(-pi/2);
    AddPoint(3*pi/2);    AddPoint(-3*pi/2);
    AddPoint(5*pi/2);    AddPoint(-5*pi/2);
  with Chart1FuncSeries2.DomainExclusions do begin
    AddRange(NegInfinity, 0);

Now the program runs fine.


Cleaning up


Before we finish we could apply some improvements. Never show several curves in the same chart without a legend, there would be no way to distinguish them. Enter the function names as Title of both series, and set the legend's Visible to true.

Additional coordinate axes (TConstantLineSeries)

One more small improvement: Plots of mathematical functions often have axes crossing at the origin. The trunc version of TAChart now has an option to shift the axes away from the chart edge by applying the new property Position. But maybe you prefer the release version 1.0? For this case let's go another way:

Open the series editor again, and add two "constant line" series. These are very simple series that run parallel to the coordinate axes. Their location is defined by the property Position, their direction - horizontal or vertical - by LineStyle. Since Position has the default value 0 there is nothing to do there. You just have to set LineStyle of the one series to lsHorizontal, and that of the other one to lsVertical. The TConstantLineSeries also has an arrow which can be activated by setting Arrow.Visible to true. Play with Arrow.Length and Arrow.BaseLength to find the shape that you want.

Finally you should turn off the constant series' property Legend.Visible, otherwise there will be two unidentified entries in the legend.

Here's the final result: