mark points in a graphic!

Hi again!!
i have to plot two diffrent functions in the same graphic.
I got that each time i click in one pint it's marked...but the problem appears
when i plot the second function and i want to do tha same.
Each time i click point 0 of graph 1, the two points 0 of graph 1 and 2 are
marked,and i'd like only the one i click!
i understand why it happens...because this is my code:
n = GTK_PLOT_CANVAS_PLOT(child)->datapoint;
        printf("Item selected: DATA\n");
        printf("Active point: %d -> %f %f\n", 
                GTK_PLOT_CANVAS_PLOT(child)->datapoint, x[n], y[n]);
                
                
             
       
gtk_plot_data_add_marker(dataset[0],GTK_PLOT_CANVAS_PLOT(child)->datapoint);
       
gtk_plot_data_add_marker(dataset[4],GTK_PLOT_CANVAS_PLOT(child)->datapoint);
       
but...how can i say to the computer, that datapoint 1 of graphic 1 is different
of datapoint 2 of graphic 2??

please...can you answer me??
is very important!

Thanks a lot





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