Allow advanced field rendering for chart views

In most cases Charts just fetches the value from views fields directly rather than rendering the field. I can see why this might be the case (in fact I broke my chart immediately when I tried to change it due to adding a % sign in my rewrite) but there are some situations where views rewriting is necessary. In my case I was applying a twig filter to an aggregated result to remove the decimal places.

I'd like to have the option to explicitly enable rewriting for when you do need to use it on a chart view. I'll attach a patch shortly.