New Post: How can I convert pixel coordinate to map coordinate for a gridline?

You have to use pixel coordiates for g.DrawLine.
In your example you'd simply use

g.DrawLine(bluePen, ptf1, ptf2);

because that are the pixel coordinates that correspond to the geographic cooordinates (ptfpr1, ptfpr2) you wanted to use for drawing.

            System.Drawing.Point ptf1 = new System.Drawing.Point(0, 0);
            System.Drawing.Point ptf2 = new System.Drawing.Point(300,400);
     
            DotSpatial.Topology.Coordinate ptfpr1 = new DotSpatial.Topology.Coordinate(map1.PixelToProj(ptf1));
            DotSpatial.Topology.Coordinate ptfpr2 = new DotSpatial.Topology.Coordinate(map1.PixelToProj(ptf2));
            g.DrawLine(drawingPen, ptf1, ptf2);