In addition to Jon's solutions, you might also want to check:
Interactive Chart http://www.tushar-
mehta.com/excel/software/interactive_chart_display/index.html
or if you want to try to match the smoothed curve Excel draws arbitrarily through
points, google the groups for 'Dave Braden' and 'cubic spline', or go to Brian
Murphy's site, and look for the Excel Stuff link, which includes a treatment of the
bezier function used by Excel to draw this curve: