Add a period's trend to (#277) period comparison

[divinerites]

Prerequisites

• I have searched open and closed issues to make sure that the feature has not yet been requested.

Feature request

If/when #277 is done, being able to see the trends in those 2 periods would help us understand the main (simple) trends and the trend differences between those 2 periods

How would you like it to work

No need for complicated stuff, a simple regression would be enough in the majority of use cases.