Are you approximating this as a function or just the points? If just the points, then I take it you assume the graph has a straight line between points.

In that case y(2.7) = y(2)*.3 + y(3)*.7

y(1.5) = y(1)*.5 + y(2)*.5

And the integral between y(d) and y(e) is just (y(d) + y(e))*(e-d)/2

So you would just do that for each piece and sum them.

In that case y(2.7) = y(2)*.3 + y(3)*.7

y(1.5) = y(1)*.5 + y(2)*.5

And the integral between y(d) and y(e) is just (y(d) + y(e))*(e-d)/2

So you would just do that for each piece and sum them.