i think `extents` is a bit misleading as a name here, traditionally i think of an extent as being just the minimal cover of a dataset. so for `["a", "b", "c"]` the extent is `["a", "b", "c"]` but for `[0, 1, 2, 2.1, 3]` the extent is `[0, 3]`.

in this case, what you're calling the extent is actually just the perspective's values