XPOSN(expr)

Returns X coordinate value that is 'expr' * 100 percent of the
X axis range.  Plot scaling must have previously been done by
a PLOT or NOTE procedure.

Used for positioning a note when autoscaling is done; e.g. for
a note that starts at 10% of the X axis range and 90% up the
Y axis:  

     NOTE(XPOSN(.1),YPOSN(.9));
