Make circular plots actually circular

Also update tutorial plots to match.

Signed-off-by: Mark Hymers <mark.hymers@ynic.york.ac.uk>
1 job for 45-tidy-up-root_plot in 1 minute and 49 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build Test
passed #68616
build_test

00:01:49

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