Choropleth selectors: one region at a time, in the order the trace declared
them.
Plotly draws a path per calc entry — including the entries it resolved to no
region at all, which stay in the DOM with no shape — so a region is
addressed by its own position rather than by counting the ones before it.
That keeps the pairing right for a map whose data has holes, which is the
ordinary case: a value the source had nothing for is still a row.
The trace's group is counted within its own geo subplot, since plotly hangs
no uid class on these groups either.
Choropleth selectors: one region at a time, in the order the trace declared them.
Plotly draws a path per calc entry — including the entries it resolved to no region at all, which stay in the DOM with no shape — so a region is addressed by its own position rather than by counting the ones before it. That keeps the pairing right for a map whose data has holes, which is the ordinary case: a value the source had nothing for is still a row.
The trace's group is counted within its own geo subplot, since plotly hangs no uid class on these groups either.