Thanks for your reply @tomhollander.
I might be missing something here. I’ve added a computed field AllProjectNumbers to an individual Chart with the aggregation:
{
$setUnion: [
"$NestedArrayA.ProjectNumber",
"$NestedArrayB.ProjectNumber"
],
}

When I try to add a Dashboard filter- I might not have been clear it was a dashboard filter I want to add. My computed field itsn’t an option and I have no option to add “missing” fields.
The reason I’d like to use a Dashboard filter is to allow users of the Dashboard to select a Project Number to be used in a number of charts.