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Unit: SDL_rchart
Class: TContourPlot
Declaration: OnBeforeRenderData: TRenderEvent;
{ TRenderEvent = procedure (Sender: TObject; canvas: TCanvas; Top, Left: integer) of object; }

The events OnBeforeRenderData, OnDataRendered and OnScalesRendered  provide hooks for adding user defined graphics to the contour plot. Internally, a chart is constructed in three phases before the result is copied to the screen: in the first phase the scales are drawn, then the data is drawn and finally the crosshairs are rendered. The OnScalesRendered event is triggered after the scales are drawn giving access to the non-data areas of TRChart (i.e. the areas where the scales and captions are drawn). The events OnBeforeRenderData and OnDataRendered are triggered before and after drawing the chart. Note that the latter two events give access to the chart canvas (the area where the chart elements are is drawn).

The event OnBeforeRenderData occurs immediately before the chart is drawn. The variable parameter Canvas provides access to the canvas of the data area. The parameters Top and Left contain the offset of the data area relative to the entire chart area. You need these two values if you want to position your own drawing elements relative to the real-world coordinate system (by using the method R2M).

Hint 1: Graphic elements outside the data window are cut automatically

Hint 2: Please note that drawing graphic elements in the OnBeforRenderData event does not create an entry in the data container. Thus you are at your own as far as the management of these drawing elements is concerned.

Hint 3: In order to avoid unwanted size effects regarding characters displayed on canvases of different resolution (i.e. the screen and a printer) you should never directly assign the font size within the event. Use SetCanvasFontSizeScaled instead. So, for example, the statement Canvas.Font.Size := 12; should be replaced by SetCanvasFontSizeScaled (Canvas, 12);

Example: Following is an example on how to implement user defined graphics which is displayed below the RChart data. It displays the text HALLO twice, one text is positioned at absolute coordinates (120,120), the other text is drawn relative to the world-coordinates at position [20.0, 1.0]. Note that the method R2M returns coordinates relative to the chart window (and not relative to the data area); you have therefore subtract Top and Left before positioning the graphics. An example how to use the OnBeforeRenderData event is included in the sample program CHARTPOLYGONS.DPR which can be downloaded from the SDL Web site.

procedure TForm2.RChart1BeforeRenderData (Sender: TObject;
                           Canvas: TCanvas; Top, Left: integer);

  xout,yout: longint;

canvas.Font.Color := clBlue;
Canvas.TextOut (120,120,'HALLO');
RChart1.R2M (20.0, 1.0, xout,yout);
Canvas.TextOut (xout-Left, yout-Top,'HALLO');

Last Update: 2023-Dec-13