The tooltips shown when hovering over the learning history graph are displayed relative to the page position rather than to the graph, so they appear in the correct position only before scrolling down. I’m running Firefox 49 on Fedora 24.
Scrolled down:
Top of page: http://community.memrise.com//uploads/memrise/original/2X/d/d75dfc7cd328e07b53b7dfd8904896ce22b566e9.png