For example, I have points
Y X 100 50 90 43 80 32
need to solve for y = 50
or
Y X 1/1/2009 100 1/3/2009 97 1/4/2009 94 1/5/2009 92 1/6/2009 91 1/7/2009 89
need to solve for y = 1/23/2009