Suggestion: Excel-style grouped axes

[wurli]

Excel has really nice support for 'grouped' axes like the following:

Currently a similar effect would be achievable using bracketed ranges, but I wonder if it's worth considering a dedicated feature, e.g. which would allow the user to specify columns to use for the inner/outer axis levels, and perhaps render the results similarly to Excel (instead of simply being an alternative API for bracketed ranges).

Discussion on Fosstodon: https://fosstodon.org/@teunbrand/111839196732828384

Really excited to find out about this package - looks like an amazing addition to the ggplot2 extensions ecosystem :)