Add validation/constraint for statewide incarceration rate calculation

[hobuobi]

Currently, we do not place any constraint on the month over month values for the Statewide Incarceration Rate. This can result in the The proposed solution would be to create a floor for the % of the population (not % counties) represented in the calculation for any given month; data points below this floor would be omitted from the graph. The floor should be based on realistic numbers, and could be derived in a few ways (non-exhaustive):

• 2SD below average % population represented across all states
• 2SD below average % population represented in the state of interest (across all data points)
• 2SD below average % population represented in the state of interest, only in the given time range selected in the graph.

[hobuobi]

Update: Chatted with @colincadams and I think we can set an arbitrary floor for this in the meantime (e.g. 10% population covered). We will do more research into the population coverage later and pick a more informed floor!

This ticket should now be actionable!

[nasaownsky]

Closed by #106