How does IDL calculate the graph axis values?

[Declan Vogt <drv102(at)ohm.york.ac.uk>]

I'd like to find out what IDL will use as the max and min values for a
graph axis before it actually plots the axis. I've been doing this by
creating a window, drawing the axis, and reading !x.crange, then erasing
the window, but it's not very elegant, and it doesn't work for
postscript.

Does anyone know if there is an IDL routine I can call?

I need to do this, because I am trying to polar plot negative values,
which I have to do by adding an offset to make them positive, and then
drawing the axis values myself. But to start with, I need to choose an
axis range, and since IDL does that so nicely, I'd like to use its code.

Any hints?

Declan