Just a couple of physics notes: I wrote r as a scalar. This is the magnitude of the vector r.; The 1 over 4πε 0 is just a constant.; Electric potential is a scalar quantity, not a vector.

The same data used with this PLOT statement in the GCONTOUR procedure produces a contour plot like the one in Line Contour Levels: plot y*x=z / levels=-7.5 to 7.5 by 2.5; The contour lines in Line ...