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Two x axes per plot

David Fanning has a nice illustration of how to use a linear and
a log 'y' axes in the same plot. I have a question that is related
to this, but not quite the same. How does one have an upper and
a lower x axes, where the lower one in linear and the upper one
has *some selected* values of a function of x. 

The example that comes to mind is the typical Arrhenius plot of
y = A*exp(-T0/T), where one plots ln(y) vs 1/T to show the
exponential nature of the data. This can be accomplished with

plot_oi, 1/T, y, ....

Now, how does one plot an upper 'x' axis which gives the 
corresponding values of T at "convenient" values. Say, that

100 < T < 500

so that the lower axis goes from 0.002 to 0.010. The upper
axis should have tick marks and the values at, say, 100, 200,
300, 400 and 500. How does one position the tick marks? Is it
at all possible?



Surendar Jeyadev         jeyadev@wrc.xerox.com