fix fill is being filled even in yet unplotted areas #466

Job	Run time
build-macos-brew (clang-9)	2m 45s
build-macos-no-brew-Qt6 (clang-9)	5m 0s
build-linux (gcc-9)	7m 41s
build-linux-Qt6 (gcc-9)	7m 59s
build-macos-brew-Qt6 (clang-9)	3m 39s
build-macos-no-brew (clang-9)	5m 10s
build-windows-mingw (5.15.*)	10m 6s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	10m 37s
build-windows-vs (5.15.*)	8m 18s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	12m 37s
	1h 13m 52s

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