Graph, gnuplot and Scilab

That is some good software there. I do not feel so bad now. I am not sure about his explanation of no sign changes.

y - x^2 = 0 has no sign changes either but graphs properly here. Or maybe you do not notice it because only (0,0) is left out!

]]>I have found this is a common problem in many open softwares including Graph, gnuplot and Scilab.

Maybe **this page** will be helpful to you.

I agree with you. It's difficult to write a contour plotter from scratch.

Many thanks to those kind people who make open/free plotters for us.

Thanks for that, I will look into it.

]]>Mathematica agrees with you. I remember trying to write my own contour plotter. In two different languages! I do not think I ever really succeeded.

I tried to do it by solving for the points. That is difficult enough when you are standing over the computer and watching every move. In a plotter the program has to do it all by itself. Sometimes they are going to miss some and sometimes they will add a few.

By the way that is a clever way to draw two squares.

]]>The MathsIsFun Equation Grapher gives wrong graph for

(abs(x-1) + abs(y) - 1) * (abs(x) + abs(y-1) - 1) = 0.]]>