I need to interpolate two points (x0,y0) & (x1,y1) to find the y value for a
given x. First, I need to find x0 and x1 from a column using a lookup
function. The x's column increases as you go down.

For example given a data set:

x's y's
20.33 5
22.69 7
25.06 8
27.42 18
29.79 37

Say I want to find y for x=23. I want to lookup the two closest values of
x's that x=23 is between(22.69 and 25.06) and then use these two points to
linearly interpolate the y for x=23. Any suggestions?