Fix 'no shared levels' warning

[matt-dray]

Noticed in #120 that there was a warning message being printed to the console. This happens on main, so it must have been introduced by #107. Not sure how I didn't spot this before.

Warning: No shared levels found between `names(values)` of the manual scale and the data's colour values.
Warning: No shared levels found between `names(values)` of the manual scale and the data's alpha values.
Warning: No shared levels found between `names(values)` of the manual scale and the data's colour values.
Warning: No shared levels found between `names(values)` of the manual scale and the data's alpha values.

Happens after switching to a different geography. And then after every time switching back and forth.

Seems related to the rates and funnel plots.

Should investigate and fix, as maybe it's messing with data presentation.

[matt-dray]

I checked out tags v0.3.4 (last release) and v0.3.3 and this problem was there too. I'm pretty sure it wasn't there at the time those releases were made. So might be related to a recent update in a dependency?