how can I find actual axis limits for an existing plot?

I would like to query an existing plot to learn the actual x and y 
limits of the plottable area.  Is this possible?  I can discover the 
extreme tick mark locations but not the actual plot area limits.  
Setting the limits with ylim will not help in this case because I am 
overplotting an existing plot with a separate function, and I don't want 
my end-user to have to pass parameters around.

Thanks,
Mark