How to sample data from a plot to a function.?

Hi evryone, Can you tell me if there is any way to obtain a discrete set of data points from a plot and use those data points later in a function. Suppose i hv a function 'Flow_inlet' in which i hv to feed data of Outlet flow at diff time instants and i have a plot of Output flow vs. time. Now how can i feed the data from the curve to the function. If it cud be done anyway then how?? Plzz enlighten me.Really need ur help. Thnxx in advance!!