Server side or Javascript calculations?
This depends a lot on what you are trying to do. If the chart is dynamic and animated, doing it client side with js may be the only choice. It also depends on how much data you have. I would not recommend doing it in js if you have over 10mb of raw data.
In general is it better for performance to do lots of data calculations on the server side or on the javascript side? 
I have a bunch of data that i'm displaying on a page - and I'm wondering if I should format/ parse/ make calculations on that data on the server side (in python) and return a template or if I should return the data as is and do all my calculating/ formatting on the javascript side?
Are there any general rules of thumb when making these decisions?
Examples of things i'm calculating - converting timestamps to dates.