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xmitte,ymitte: integer;

xmitte:=deinbmp.width div 2;    
ymitte:=deinbmp.heigth div 2;                    

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var xmitte,ymitte: integer;
begin
  xmitte:=meinbmp.width div 2;
  ymitte:=meinbmp.heigth div 2;

  bmp.Canvas.moveto(xmitte,0);
  bmp.Canvas.lineto(xmitte,bmp.heigth);
  
  bmp.Canvas.moveto(0,ymitte);
  bmp.canvas.lineto(bmp.width,ymitte);
  Image1.assign(bmp);                    

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procedure TForm1.Button1Click(Sender: TObject);
var xmitte,ymitte,i,tw: integer;
    bmp:TBitmap;
    s:string;
begin
  bmp:=TBitmap.create;
  bmp.width:=200;
  bmp.Height:=200;
  xmitte:=bmp.width div 2;
  ymitte:=bmp.height div 2;

  bmp.Canvas.moveto(xmitte,0);
  bmp.Canvas.lineto(xmitte,bmp.height);

  bmp.Canvas.moveto(0,ymitte);
  bmp.canvas.lineto(bmp.width,ymitte);

  i:=20;
  while i< xmitte do
  begin
    s:=IntToStr(i div 20);
    tw:=bmp.Canvas.TextWidth(s) div 2;
    //positive Achse
    bmp.canvas.MoveTo(xmitte+i,ymitte-5);
    bmp.canvas.LineTo(xmitte+i,ymitte+5);
    bmp.canvas.TextOut(xmitte+i-tw,ymitte+5,s);
    //negative Achse
    bmp.canvas.MoveTo(xmitte-i,ymitte-5);
    bmp.canvas.LineTo(xmitte-i,ymitte+5);
    bmp.canvas.TextOut(xmitte-i-tw-3,ymitte+5,'-'+s);
    inc(i,20);
  end;

  bitblt(canvas.handle,0,0,bmp.width,bmp.height,bmp.canvas.handle,0,0,srccopy);
  bmp.free;
end;