Is it possible to interpolate a value for x with knowledge of y? For example, approx(x, y, xout) will give me y's given a set of x's, which is opposite to what I'm after. I've tried switching x and y, e.g., approx(y, x, xout), but in a real data set it is possible to have more than one y for a given x causing approx() to remove coordinates. Thanks for your help, Greg.