Spline Interpolation between data points in Stats

[baylessj]

Pulled from a comment on #41:

Currently I'm using an EMA filter to make a trend line for the data, but the jagged edges between the data points don't look great.

I'd like to set up spline interpolation between the data points, but finding a good way to do that without requiring a lot of data points is tricky. Found the following resources though:

• https://docs.scipy.org/doc/scipy/reference/generated/scipy.interpolate.BarycentricInterpolator.html#scipy.interpolate.BarycentricInterpolator
• https://blog.splitwise.com/2012/01/31/mystery-solved-the-secret-of-excel-curved-line-interpolation/
• https://en.wikipedia.org/wiki/Centripetal_Catmull%E2%80%93Rom_spline

From the wiki page and a couple posts on SO it seems like Catmull Rom splines are the way to go

[baylessj]

This could probably be lumped together with #265, the graphs may look smoother with that package anyways.