Truth plot does not reappear when it should for certain forecaster

At times, truth plot does not reappear when it should after not having data for selected forecasters. So far have only seen this with `imperial-ensemble2`.

1. Select `imperial-ensemble2` and another forecaster (like the baseline)
2. Select location `AK`
3. Remove other forecaster so just `imperial-ensemble2` is selected. There is no data, so no plot - fine, the error message shows up.
4. Change location to `US` (which this forecaster does have data for). Observe that truth plot is still gone. If you choose this forecaster and `US` directly on dashboard load, we see a truth plot.

Start at the dashboard's forecaster and location selection entry points, then trace how the truth plot is handled when a selection has no data. Reproduce the AK-to-US sequence with imperial-ensemble2 and compare it with loading US directly. Done means the truth plot reappears after changing to a location where data exists.

Written by the indexing model from the issue text.