This tutorial by Nathan Yau from FlowingData outlines an easy method for fitting a line to data points of a scatterplot by using open source software.
Excerpt
"Maybe you have observations over time or it might be two variables that are possibly related. In either case, a scatter plot just might not be enough to see something useful. A fitted line can let you see a trend or relationship more easily."
Sources
Nathan Yau (2014), How to make a scatterplot with a smooth fitted line, FlowingData. Retrieved from: http://flowingdata.com/2010/03/29/how-to-make-a-scatterplot-with-a-smooth-fitted-line/
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Method