I have the following chart in Excel:
I want to calculate the area under the graph line.
Any idea?



For each adjacent pair of points, take average of ycoordinates, then multiply by difference of xcoordinates, and accumulate for all pairs. 


For your related post (How to paint area under graph line with average line?) I suggested using a column chart. The area chart overemphasizes some points more than the values should allow. Since the points are discrete measurements, the sloping lines between measurements are meaningless, and calculating areas is going to be misleading. So the "area" above the values is the sum of (value  average) for value > average, and the "area" below the values is the sum of (average  value) for value < average. Very simple to calculate with two simple formulas. 

