That is something I am a bit weak on. I know you can add trendlines, but I'm not sure how they're calculated, or of that is really what you're looking for. To add one, select the chart, then go to the Chart Tools | Layout tab of the Ribbon, and select Trendline. You can play with the options here and see if they give you what you need. If not, I'm going to tag out. Good luck!