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Actually please keep yourself from using the cvWhatever functions, which are the old C-style API. You should just use the up to date C++ - API.

This code snippet should help you out, the x and y coordinate are fixed in the center point parameter.

// Long explicit code sample
Mat image = imread("location_of_image");
int x = value_x;
int y = value_y;
int radius = value_radius;
// Draw circle
// The scalar gives you the color depending on the values for the BGR color channels
circle(image, Point(x,y), radius, Scalar(255,0,0));
// Show the cirle on image
imshow("result", image);