If you want to interpolate (as opposed to extrapolate), then you have
lots of options. For interpolation between close points, you could use
linear interpolation. For more distant points or more critical
applications, you could use fit a polynomial or a ratio of polynomials,
or some other easily fit nonlinear function.
The more you know about the relationship, the better you can tailor the
method. Extrapolation is more uncertain unless you know something about
the form of the relationship.
Excel's chart smoother appears to use Bezier curves. Brian Murphy has
posted code for Bezier curves.
http://www.xlrotor.com/excel_stuff.htm
In most cases, the difference between Bezier curves and cubic splines is
minimal. An example where the difference is noticeable is
http://www.google.com/groups?threadm=3E7291CA.7020907@no_e-mail.com
David Braden has posted code for cubic splines
http://www.google.com/[email protected]
Jerry
