`PolygonSeries` and `PointSeries` do not display time on the x-axis properly

[C-Loftus]

When displaying time on the x-axis for either a PointSeries or a PolygonSeries, it appears that the value becomes duplicated. This is happening in the covjson playground without editing the PolygonSeries example at all. It also happens locally in a test application using pygeoapi which uses covutils to render the leaflet chart.

[C-Loftus]

And apologies in advance if I have misidentified which repo is causing this behavior. I am not sure if the error is coming from the parsing of covjson via this repo, the rendering of the graph via https://github.com/Reading-eScience-Centre/leaflet-coverage, or somewhere else.