I see the graph. When I first read it, I instantly inferred from that graph that they simply were adjusting to the lichess platform because (1) everybody starts with a rating of 1200 on chess.com and 1500 on lichess--the two biggest chess playing sites by far--so you naturally 'raise your rating' if you're simply better than the website's guess at average and (2) it just takes a bit of adjustment to a new platform.