Iris to penguins

[baldwint]

Maybe someone is already working on this, but to help with #2213 I found it fun to rewrite three of the (currently broken) compound chart documentation sections using the new penguins dataset instead of iris.

I used the mapping

{
    "sepalLength": "Beak Length (mm)",
    "sepalWidth": "Beak Depth (mm)",
    "petalLength": "Flipper Length (mm)",
    "petalWidth": "Body Mass (g)",
    "species": "Species",
}

since this seemed more or less faithful to the patterns in iris. I also chose to un-anchor the scales from zero since the penguin means are not very close to zero.

While working on this, I found it useful to make a modified version of vega_datasets that retains some of the older datasets so that the docs can build without failing on examples that haven't been updated yet.

[jakevdp]

Thanks! This will have to wait for a vega-datasets release that includes the penguins data.