Include uncertainty bands for forecasts with 20% and 80% predictions

We show the point forecasts in the observed plot. We can include the 20% and 80% forecast values to show uncertainty bands. Note: this will be tricky when showing forecasts for total over all locations because we cannot just do a simple sum of the values anymore.

Other fancy idea: clicking on a point in the main plot shows the forecast data for that date in the observed plot

No files or tests are named. Start by locating the observed-plot rendering and the code that combines forecasts across locations; inspect how point forecasts and the 20% and 80% values are represented. Done means uncertainty bands appear for forecasts, with a defined approach for totals across locations; the clickable-point idea should be treated as separate scope.

Written by the indexing model from the issue text.