I have two datasets that I would like to plot in a single figure.  The first
plot is generated by a function that then takes a subset of the data.  (It
is biological data so it is usually by chromosome e.g.
function(data1,subset="chr8") ) Since not only are the chromosomes different
sizes, but across different datasets there may be different numbers of
points for a single chromosome, I do not know the coordinates on the x-axis
prior to running the function.

I want the scale of the second plot to align to that of the first, but I
don't know how to find the default xlim for the graph generated by the
function.  Is there any way to access this information so I can then pass it
to the plot() of the second plot?  Thanks!

- Fincher

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