IIRC in another question, your said your Point class actually uses `float x,y;` for the coordinates. So one thing that can help is to shift the centers (the "zeros") over to the edge of the pixel, like:

    int xi = pt.x + 0.5;
    int yi = pt.y + 0.5;

And use those integers in calls the GLUT line drawing api.

I think this change would go inside your `drawLine()` function.