I've having a problem creating multiple x-axis and I'm hoping someone

here will be able to help me. I have two directly correlated values

(z and N) that I am using as the independent variables. What I would

like to do is plot my data vs. N, but then show the corresponding z

values on the top axis (there is a 1-to-1 correspondence between every

z and N value). I have tried twiny, but this requires me to plot the

line again and, since the z and N values are scaled differently, I end

up with 2 lines that don't match up.

I hope that was understandable. Any advice? Thanks.

-Jeffrey