Oh, I see. You'll need Coulomb's Law then.
http://en.wikipedia.org/wiki/Coulomb%27s_law
q1 and q2 are the absolute values of the charges of the ions - in both cases, 2 fundamental units (with one fundamental unit being 1.602 × 10^−19 coulomb). This will give you an equation of the form of
F = c/r
with c being a constant equal to kq1q2.
This sort of graph should look something like the first one on this page:
http://id.mind.net/~zona/mmts/functi.../oneOverX.html
F would be the vertical, or y, axis, while r would be the horizontal axis. Of course r goes from one positive value to another so you will only show part of the positive part of the graph.