Let's say, for arguments sake, that the latest figures showed that the UK had 100,000 total confirmed cases of Covid-19.
Setting the above field to 50 would mean the retrieval would bring back all countries where the latest figures showed that the number of total confirmed cases was between 50,000 (i.e. 50% of 100,000) and 150,000 (150% of 100,000).

The lower this figure, the fewer countries should be displayed.
Conversely, and going back to our example . . . if the percentage was set to 99% (the maximum) then this would retrieve all countries where the latest figures showed that the number of confirmed cases was between 1,000 (1% of 100,000) and 199,000 (199% of 100,000), which is a much larger window, and so the retrieval should return far more countries.

The default value will vary but is set to ensure that only seven or eight countries are retrieved, thereby reducing unnecessary clutter in the graph.

Conscious that sometimes lines can be rendered in a very light yellow colour, making it difficult to distinguish from the white background . . .

Reducing the colour will darken all datasets.
Set this to 0 if you want to see all datasets rendered in black.
Set to 255 to ensure maximum colourful, rainbow-ness.