Search the google.com archives of the XL NGs to find code that will
give you how XL computes its smoothened line connecting the points
(Dave Braden's cubic spline code and the bezier curve courtesy
xlrotor.com). Then, use calculus with that equation.
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Regards,
Tushar Mehta www.tushar-mehta.com
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If you know the equation of the curve and if you don't like calculus, you
could use Solver to find the max. I would be tempted to give some
constraints for the x-value