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delphiteechartdelphi-10-seattle

Is an exponential scale of an axis with TDBChart possible?

Is it possible to have an exponential scale of an axis with TDBChart (instead of a logarithmic scale)?

A logarithmic scale of the y-axis is useful, when the graph corresponds to an exponential growth. As the following example shows, the y-axis values of the graph between 0 and 1 are extra highlighted while indiviual outliers beyond that will become more and more unimportant:

However, if there is somekind of an inverse behavior of a graph compared to above example, where small values aren't very important (noise) but indiviual outliers should be made explicitly visible, then an exponential scale is useful:

So, is it possible to scale an axis with TDBChart exponentially?

Solution

  • The following code populates a series with exponential data:

    uses Series, Math;

procedure TForm1.FormCreate(Sender: TObject);
var i: Integer;
begin
  Chart1.View3D:=False;
  Chart1.Legend.Visible:=false;

  with Chart1.AddSeries(TLineSeries) as TLineSeries do
  begin
    Pointer.Visible:=true;
    Pointer.Size:=2;

    for i := 1 to 10 do
      Add(Power(2, i));
  end;
end;

    The result looks as follows:

    exponential growth

    Then, if you add the following code to the above:

      Chart1.Axes.Left.Logarithmic:=true;

    Now the data is still the same but the left axis scale changes to a logarithmic scale:

    logarithmic scale

    You can also change the labels format, ie:

      Chart1.Axes.Left.AxisValuesFormat:='00e-0';
  Chart1.Axes.Left.LabelsExponent:=true;

    exponential labels

    Edit:

    Setting a logarithmic base of 1.544 on TeeChart and custom labels to show those labels in your screenshot:

    uses Series, Math;

procedure TForm1.FormCreate(Sender: TObject);
var i: Integer;
begin
  Chart1.View3D:=False;
  Chart1.Legend.Visible:=false;

  with Chart1.AddSeries(TLineSeries) as TLineSeries do
  begin
    Pointer.Visible:=true;
    Pointer.Size:=2;

    for i := 1 to 7 do
      Add(Power(2, i));
  end;

  with Chart1.Axes.Left do
  begin
    Logarithmic:=True;
    LogarithmicBase:=1.544;
    MinorTicks.Visible:=false;

    with Items do
    begin
      Clear;
      Add(0.1, '0.1');
      Add(1.2, '1.2');
      Add(1.5, '1.5');
      Add(2, '2');
      Add(3, '3');
      Add(5, '5');
      Add(7, '7');
      Add(10, '10');
      Add(15, '15');
      Add(20, '20');
      Add(30, '30');
      Add(40, '40');
      Add(50, '50');
      Add(70, '70');
      Add(100, '100');

      SetMinMax(0.1, 100);
    end;
  end;
end;

    It looks like this:

    LogarithmicBase 1.544

    The distance between 0.1 and 1.2 is much greater than in your screenshot, and I'm not sure if that's a bug or if that's mathematically correct.
    I can change the axis scale to start at 1 instead of 0.1 to make it look much similar to your screenshot but I'm not sure if that would be what you want:

          SetMinMax(1, 100);

    Axis Minimum 1)