Ability to compare two sets of Filters

A little bit related to #199 - I would love to be able to compare and chart two filters, like we can for dates.

Scenario:

• look at traffic for page /landing-page-1 (filter to just this page, look at change over time on the chart)
• look at traffic for page /landing-page-2 (filter to just this page, look at change over time on the chart)

I frequently screenshot these different charts, and try to overlay them using other tools so I can compare the trends between them over time