The graph looks at the average height of the 4 Wimbledon semi-finalists each year in the Open era from 1968 - 2019.

Prior to 1986, the average height of the 4 Wimbledon semi-finalists was always under 184 cm. When the 191 cm tall Boris Becker won in 1985, this marked the beginning of an era of dominance by tall players. Between 1986 and 1998, the average height of the 4 semi-finalists was always over 184 cm, usually over 187 cm and often over 190 cm. And between 1985 and 2001, 4 different players taller than 190 cm won the Wimbledon title with the 196 cm tall Richard Krajicek being the tallest ever Wimbledon winner in 1996.

From the end of the 90s to the late 00s, relatively shorter players dominated Wimbledon. This shift occurred even though the actual titles in these periods tended to be won by two 185 cm tall players, Pete Sampras and Roger Federer. The graph would seem to indicate that Sampras was generally facing and beating taller players than himself in the latter stages while Federer was usually facing opponents of similar of shorter stature than himself. 

Recently however, things have changed and dominance of tall players has returned. Although, there's only been 1 champion over 190 cm tall (Andy Murray) in the last 10 years, the average of the 4 semi-finalists has become even taller than in period from the mid 80s to late 90s.