Can we make isoglosses?

[Emma May Smith]

I asked around on Reddit, and someone answered with this very useful map. The pink/purple marks are areas where the /l/ in "Milch" assimilates with the preceding vowel.

Also this map from the Bavarian Speech Atlas which makes it clear that, at least in modern German, realisations without a liquid were common even in big cities. This map shows Upper Bavaria only, but also has commentary (in German) about the different realisations:

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(Let me know if the image isn't accessible. If somebody wants to post it here, be my guest, but it wouldn't work for me.)