I have a set of data points, ie. x = 1,2,3,4,5,6 corresponding with
y=4,17,18,22,15,7

I want a cell that will work out the expected value for y for any value (ie.
x=3.3, y=?) when the x value is provided to it from another cell. I want to
be able to change that input cell and automatically recalculate the new value
of y.

I need some kind of routines that finds the data points either side of the
required value and interpolate between them. How do I make this routine and
carry out the interpolation?