Given a series of x and y data, how can I interpolate to find y given a value
of x based only on a line between the two adjacent points in the data series?
This would be like the TREND() function, only I don't want regression of the
entire data series, just the (x,y) data points immediately above and below
the input x value.

This seems such fundamental and essential function that I've been
exasperated for years that Excel doesn't have it or directions how to find
it. I realize that Excel calculates these individual lines every time it
connects the dots in a graph, but not having a function to achieve the same
result on a worksheet is puzzling. Am I missing something?