Double tickmark annotations

["Shawn L. Young" <young(at)chaos.dartmouth.edu>]

Hello,
	I would like to know how to give more than one name to a each
tickmark along the x axis.  For instance, I want to list both a satellite's
latitude and radial distance from the center of the Earth along the x axis.
However, I don't know ahead of time how these two quantities correlate so I
can't hardwire tick names using xtickname.  Is there a way to give the plot
routine both arrays and have it determine the ticknames?  If not, the help
file is very brief on the use of xtickname.  If I knew how IDL chose where
to put the ticknames I might be able to write code to do the correlating on
the fly.  But when I am using a > 500 element array for the x axis and want
to be sure that I am doing the best correlation I can between array
elements and xtickname elements.
Thanks