How to take cairo_set_source_rgb effect for different color points?

Hi cairo developers,

I wanna draw some points indicating mouse`s X and Y, and used random
number from 0 to 255 for RGB color of the points, such as:

for loop
{
double r = Map(i, 0, 100, 0, 255); // Re-maps a number from one range to
another
http://www.openframeworks.cc/documentation/math/ofMath.html#show_ofMap
double g = 0;
double b = 255 - r;
cairo_set_source_rgb(c, r / 255.0, g / 255.0, b / 255.0);
cairo_move_to(c, x, y);
cairo_close_path(c);
}
cairo_stroke(c);

It is unable to put cairo_stroke into the for loop, it might be a heavy
operation when draw the path too often.
But cairo_set_source_rgb failed to take effect in the loop if
cairo_stroke was outside.

The source code is available via
https://github.com/xiangzhai/grt/tree/master/GRTExamples/ClassificationModulesExamples/CairoX11DTWExample

So how to take cairo_set_source_rgb effect for drawing different color
points?
Please someone give me some advice, Thanks a lot :)

Best,

Leslie Zhai

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