Potential performance issue: plotting slow in matplotlib == 3.3.0

Dear Zipline Maintainers,

I hope this comment finds you well.

I have discovered a performance degradation in the plot function of matplotlib version 3.3.0. And I notice it lies in etc/requirements_py36_locked.txt. I am not sure whether this performance problem in matplotlib will affect this repository as I don't know whether the dependency in the file are used in production or not. I found some discussions on matplotlib GitHub related to this issue, including matplotlib/matplotlib#18743.

If the performance issue really affects your repo, I would recommend considering an version change to matplotlib >= 3.3.3 or <3.3.0(e.g. 3.2.2) or exploring other solutions to optimize the performance of plotting.

Thank you!

[lhjnilsson]

Hey!
I think you should look into using https://github.com/stefan-jansen/zipline-reloaded instead