With Slavic languages it’s never a simple word-based analysis - they’re highly inflected so you need to lemmatise text first which is a bit hit or miss when done automatically. I assume author of the graph did that manually for fascism because it’s a loanword and there’s less ambiguity to account for but it gets tedious quite fast if you really get into it. I got into it once in my mother tongue (Polish) and that rabbit hole goes really deep. Here’s a brief overview of how that process looks like.