Make multivariate demo

[joewdavies]

Recreate some of those beautiful multivariate maps, with venn-diagram style legends:

These are technically possible with the current Shape/Color/Size style, but a new legend type would be required.

https://cdn.bsky.app/img/feed_fullsize/plain/did:plc:gxhy7gwsqxgvetnwrbup5civ/bafkreidztg5kqd4c75kcdaojtkpcjukbd6azgja7q3cfoflblyroj7jwp4@jpeg

https://cdn.bsky.app/img/feed_fullsize/plain/did:plc:icidflhftfldb4kcfx36kncc/bafkreibpaljeygnbsgm464bm6bxj2wimnww54rb5svuskte6pvpfh3zfyi@jpeg