I do not trust any "livability" statistic that lists Frankfurt as the most livable city in Germany.
Also, the chart does not give its selection criteria. Medium sized cities to cities on the smaller side are completely missing. I get not including towns, that would overwhelm the graph.
Then again, what even is a city, what sets it apart from a town? Different regions in Europe have vastly different definitions of that, with the UK's definition being particularly notable for how useless it is.