Here are two ideas about finding the area under a curve. Because the signals are digitized, the calculations may be a little different than a traditional "area" calculation.
Consider the original function:
Right click on the Y axis, and select "Processing -> Cumulative: Forward". Each value in the curve is added up. The last value is the sum of all values.
For a digitized signal, the sum of values is considered to be the area (I think).
Or you could use a Double X cursor and do a RMS calculation. Some consider the RMS of a curve to be equivalent to the area:
1. Right click on display and choose "Add Double Cursor -> X"
2. Right click on Double X cursor and select "Calculations -> RMS"
3, Make sure double cursors are positioned over frequency range of interest
Some other articles on RMS and Double X cursors:
Hope this helps.