ENH: errorbar in qt figureoptions#18650

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andyfaff wants to merge 4 commits intomatplotlib:mainfrom

Commits on Oct 4, 2020

ENH: errorbar in qt figureoptions
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Commits on Oct 10, 2020

MAINT: refactor flatten
andyfaff
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Commits on Oct 12, 2020

MAINT: figureoptions restrict to errorbar
andyfaff
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Commits on Oct 17, 2020

MAINT: cbook.flatten
andyfaff
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