Fixes to Date Range selection

[techrah]

Based on peer feedback:

• Investigate date range validation (from less than to). It doesn't seem to be possible in Dash to have a callback update the same control that triggered the callback. Further investigation required.

• Alternate solution: move logic into data wrangling for chart. If from-date is less than to-date, swap in wrangling code

[techrah]

UPDATE: More details on the issue.

The reason that validation in the dropdown control was not implemented in the first place was because Dash callbacks cannot handle inputs and outputs being the same control.

https://community.plot.ly/t/same-input-and-output-in-a-callback/20002

This means that if I change the start year which calls some callback function... and it happens that the year is larger than the end year, that function cannot update the control that called it and reset the year back to a sane value.

[sreejithmunthikodu]

Thanks Ryan for the info. I implemented the alternate solution in #36

[techrah]

Reopening this issue. This does not seem to be working on the deployed app.