Support for calculated Custom fields in Widgets

Hi, I'm looking for a way to display calculated data in widgets.  In this particular case i want to display durations (Due date - Start date) as a Box & Whisker plot.

Whilst some widgets allow calculations (formula) such as the "Values" column in pivot tables, not all widgets do..  and even in pivot tables this column is always the last column..

Therefore, i need the ability to have a calculated custom field that i can select in the Box & Whisker Plot widget. 

If you have a solution please respond, otherwise please UPVOTE this functionality request.