The derivative of your chart will be the rate of change of your 'Y-axis' data.
Example: if your X data is in the range A1:A:20 and your Y data is in range B1:B20, then in cell C2 enter the formula '=B2-B1'. Autofill this down to cell C20. You will now have a new set of Y data (one datapoint less than your existing data) in column C.
You can add this to your existing chart by RH clicking on the chart, selecting 'Source Data' and add a new series with your X value range from B2:B20 and your Y value range from C2:C20.
You should now see a second series with 'peaks/troughs' where your original series has it's 'steepest' gradients.
This works best with an XY plot and if your X data values are in equal increments.
PM me if you have any other queries - I use Excel alot!
Good luck.. Yoshi.