Adding xy cursor functionality

Dear all,

I am using gtk-plot for a oscilloscope like program, where I show some graphs with sampled data.

http://www.pagelnet.de/uctools/a00009.html

Now I want to add a courser functionality, means:

I want to have two cross-hair like cursors, which have to be attached to one (or two) of my graphs.
When a cursor is selected it should follow the mouse movements in X direction. The best would be if
the center of the cross would stick to the selected graph, even the mouse makes a deviation in Y
direction.

My first idea was to use markers, and base on this, but it seems that makers are not working when my
plots are generatete with a custom function (gtk_plot_add_function).
Maybe there is a way to indicate datapoints manualy in my callback function.

any suggestion out there to help me in this matter?

regards
  Arne

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