determining slopes that exceed x
 

I have created elevation profiles of state park trails that I have GPSed.
The X is distance in miles, the Y is elevation in feet, plotted on a scatter
chart (the locations of the GPS points is not uniform). I would like to show
the areas of the trail that exceed a particular slope in a different color.
I suspect I would have to create a new data series that includes just those
areas and then add it to the chart. How do I determine which areas meet the
criteria a set forth?