Update charting guidelines based on discussions

[mmenestr]

trafficstars

A group of us met to discuss more specific guidelines for charts and how we should go about presenting charts across our products. We would now like to add some of the agreed upon guidance to our PF docs.

These are the brainstorming docs where we gathered notes from our meetings. Refer to this doc: https://docs.google.com/document/d/1b7ooQVCHuFPz_zqXDU_K9rQJ1vEsIBMivi2gn1LYkBw/edit and this doc: https://docs.google.com/document/d/1J0l2mjG2_BkvNXq1Vf3uRs_Zcv-lqKuDKJiAFVb-NW4/edit?usp=sharing