You have what looks like a decaying harmonic process. Something like

amplitude versus time.. Assume that times are in A1 through A1000. Assume

that amplitudes are in B1 through B1000. In column C2 enter =B2-B1 and copy

the formula down. Column C will show if you are ascending or descending.

In Column D3 enter =IF(AND(C3<0,C2>0),1,0)

Column D will have zeros everywhere except just after the peaks it will have

ones.

Use Tools -> Filter to see only the peaks. Plot this and measure the slope.