Grouped densities are stacked by default instead of sharing the same baseline

[joelostblom]

In Vega-Lite 5.8 and earlier, grouped densities would share the same baseline:

In the latest version of Vega-Lite, they instead stack on top of each other:

Open the Chart in the Vega Editor

This stacking behavior is undesirable as a default since it makes the distributions much harder to compare. This is why other packages such as seaborn and ggplot uses a shared baseline by default. Stacking the areas used to require the stack to be explicitly set like in this example.

I am not sure exactly which VL version this was changed in, but the correct behavior was present in Vega-Lite 5.8 (altair 5.0.1). Edit: I think it was in this PR #9018. Above you can also see that the extent of each density is different, and that is due to the bug reported here vega/vega#3815, but I am not sure if the stacking is also related to that same issue and/or possibly #9106