Is it my imagination or does the edge slope. When I was programming graphs for maths pupils (pascal or delphi) i had to make sure rounding methods for two points that defined the line were compatible. Is the slope caused by a rounding error?
Thanks for comment - that is quite a likely explanation. Would be nice to see the code and seems it should be easy to fix if program was nicely put into procedures (modules?). If anyone needs school graphs program i wrote i can email it to them (draws parabola etc)