Support multiple formulas in Trends

[samwinslow]

From a user, it would be nice to have the ability to:

1. Retain the formula and original metrics in the same graph. Adding a formula should not "reduce" the series down to the formula line but rather show it alongside. This may not be what everyone expects so the option to toggle may be important.
2. Support more than 1 formula on a graph.

Describe the solution you'd like

• Formulas displayed as discrete graph series items (i.e. you'd press Add Graph Series to create).
• By default, adding formula(s) would dim / reduce opacity of the source series but not remove them. User can always hit the ☑️ checkbox to remove unwanted series.

Describe alternatives you've considered

Additional context

Thank you for your feature request – we love each and every one!