Modify Overview Tab

I've been asked to modify the OverView Tab to include Symbol Coverage percentage along with Classes, Methods, and Lines.
Extra bonus points if I can have the graph turn red if Symbol Coverage is below 70%.
I know you can add new tabs, but is it possible to modify the existing tabs in this way?
Either way (Modify existing or add new tab), what do I need to do pull the data into the tab?

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