Delphi draw lines in a PlotGrid with canvas

Question

I am using a TPlotGrid because I want to try to put some lines in it. Look at this simple code:

procedure TForm1.Button1Click(Sender: TObject);
var a,b: TPointF;
begin
 a.X := 0;
 a.Y := 0;
 b.X := 1;
 b.Y := 5;
 PlotGrid.Canvas.DrawLine(a,b,1);
end;

As you can see here I have the PlotGrid on Tab2 and the button on Tab1. Why isn't this code adding a line to the PlotGrid? A line should appear when I click on the button (going from (0;0) to (1;5)).

I am new with Canvas and in particular with the TPlotGrid but the latter is not very popular on Google and there isn't much on the documentation (only 1 page with a simple example).

Did you look at the example in the documentation? docwiki.embarcadero.com/CodeExamples/en/FMXTPlotGrid_(Delphi) Note that all painting must be done in the OnPaint event. There is even a youtube tutorial: youtube.com/watch?v=8oM1F9r_i9Q. — LU RD
I guess that in the questione he was talking about that example (on the doc). I had found those 2 sources and an article as well but actually there isn't much around — Alberto Miola

Tom Brunberg Tom Brunberg · Accepted Answer · 2016-10-11T22:51:31

TPlotGrid provides a canvas with the gridlines but it has no means of storing your drawing elements (lines, rectangles etc.). The first thing you need to do, is arrange for storage of data. In your example move the points a and b to the private section of your form so they are available at any time.

In a real application you would store the elements in some data structure elsewhere.

In your button OnClick event set values to those points and call PlotGrid.Repaint;:

procedure TForm4.Button1Click(Sender: TObject);
begin
  a.X := 0;
  a.Y := 0;
  b.X := 300;
  b.Y := 100;
  PlotGrid1.Repaint;
end;

Create an OnPaint event for the TPlotGrid, here you do the actual drawing:

procedure TForm4.PlotGrid1Paint(Sender: TObject; Canvas: TCanvas;
  const ARect: TRectF);
begin
  Canvas.Stroke.Color := TAlphaColors.Chocolate;
  Canvas.DrawLine(a,b,1);
end;

Sample image with the previous code:

Delphi draw lines in a PlotGrid with canvas

1 Answers