Kajkavian-speaking Croats plot very close to Czechs and Slovaks, just somewhat more south of them, which is really interesting considering how they don't border at all and there is Hungary, Austria and Slovenia between them.
https://i.imgur.com/nWIWjKe.png
Therefore together with Slovenes they should be seen as more of an extension of western Slavs in the south, than south Slavs proper.

