The key a ridgeline's <Area>s draw: each group's density with its ridge's
offset added.
Every <Area> of a ridgeline draws the same key, over its own group's
rows, so the first one names it. Read so the ridges can be placed where
they are drawn: the offset that key adds is how far up the page each ridge
sits, and that is the order RidgelineTrace walks them in.
Depth-first for the same reason the other walks are, and stops at the
first <Area>. A tree with none, or an <Area> whose dataKey is not a
string, answers undefined, and the groups are then read in the order they
first appear in data.
The key a ridgeline's
<Area>s draw: each group's density with its ridge's offset added.Every
<Area>of a ridgeline draws the same key, over its own group's rows, so the first one names it. Read so the ridges can be placed where they are drawn: the offset that key adds is how far up the page each ridge sits, and that is the orderRidgelineTracewalks them in.Depth-first for the same reason the other walks are, and stops at the first
<Area>. A tree with none, or an<Area>whosedataKeyis not a string, answersundefined, and the groups are then read in the order they first appear indata.