I'd like a linear interpolation function that calculates the linear interpolation based on a data set hardcoded within the function itself - rather than data present in a worksheet table.

For instance given the following:
X	Y
1	8
2	5
3	2.5
4	2
The function call =interpolate(1.5) would give a result of 6.5, =interpolate(3.25) would give 2.375.

How would I perform this?