Looking at the Grafana Play demo site it does seem that when you view a time series graph in full screen, although the graph panel itself is sized correctly for the screen, the outer layout (class react-grid-layout) is for some reason sized larger than the screen resulting in an unnecessary scroll bar and the gap below the graph as reported by the OP.
See here:
https://play.grafana.org/d/000000012/grafana-play-home?orgId=1&viewPanel=2
Maybe this is an issue which should be reported here?