OptionalfillThe group's name as the segmented-style producers write it.
z is the field this schema asks for and the one to prefer, but the
violin bindings emit fill and no z at all -- the shipped
examples/violin.html is one -- so a reader of those charts was shown a
blank Group column and heard "326 (undefined)" for every extreme. Read as
a fallback rather than replacing z, since a producer may send either.
OptionalmeanMean value for violin plots when mean display is enabled.
Data point for boxplots containing quartiles, min/max, and outliers.