actually, if you compare your figure with the graph I uploaded you see the difference... I would like to plot on the x-axis 1/T going from low value on the left (high T) to high value on the right (low T). yes, it would be nice to match on the upper x-axis the T corresponding to the 1/T value on the lower x-axis. this might not be possible in mathematica, I think the x-axis always should go from low to high values towards the right. if anybody knows, please assist. laurens

Plot[Sin[1/T], {T, 1, 100}, Axes -> False, Frame -> True, FrameTicks -> {{Automatic, Automatic}, {Automatic, Transpose[{#, 1/#}] &@ DeleteCases[FindDivisions[{1, 100}, 10], 0]}}, FrameLabel -> {{"rho_xx (Ohm-cm)", ""}, {"T (K)", "1/T (1/K)"}}, BaseStyle -> 18]

actually you are right. the 1/T axis data points do increase from left to right.

ultimately, I want to fit the (natural logarithm of the) data rho_xx on the y-axis as a function of 1/T, not as a function of T. The upper x-axis is great, but can it also be used for fitting data?

I also have defined some functions that I want to plot as a function of 1/T, not T. I guess you can only achieve this by taking the function and replacing T with its reciprocal 1/T (variable substitution).

laurens