Excel does have some basic regression functions. You might be interested in the TREND() and LINEST() functions which you could apply to the linear portions of your chart. There is also a regression analysis tool in the Analysis Toolpack that I have never used.

There is also an 'Add Trendline' command in one of the menus when you have a chart data series selected. This can put the fit equation in a textbox for you, but the major problem with this method is that it fits the entire data series.