This routine uses the PxlHorz function to invert the graph screen.
The graph screen is inverted as a result of this program.
i - a counter variable
size - the size of the graph screen window in pixels
All of these are locally declared.
: PxlHorz i,-1
The -1 at the end of PxlHorz is a flag that tells the system to invert all pixels along the line. I chose PxlHorz because this requires less lines to be drawn than PxlVert does.