[query/ggplot] Replaces cross-product legends with grouped ones

[iris-garden]

Currently, hail.ggplot.geom_point generates cross-product-style legends when the shape and color aesthetics each have multiple groups. This change makes that behavior consistent with ggplot2's implementation, producing legends where the groups are factored out. Because the strategy used to accomplish this does not preserve the interactive features of the legends, such as toggling groups' visibility, this change also makes all plots generated via hail.ggplot.geom_point static.

For example, before this change, this code:

import hail as hl
from hail.ggplot import ggplot, aes, geom_point
ht = hl.utils.range_table(10)
ht = ht.annotate(squared=ht.idx ** 2)
ht = ht.annotate(even=hl.if_else(ht.idx % 2 == 0, "yes", "no"))
ht = ht.annotate(threeven=hl.if_else(ht.idx % 3 == 0, "good", "bad"))
fig = (
    ggplot(ht, aes(x=ht.idx, y=ht.squared))
    + geom_point(aes(color=ht.even, shape=ht.threeven))
)
fig.show()

Generates this legend:

After the change, this legend is generated:

Custom labels can be used by updating the code like so:

...
from hail.ggplot import ggplot, aes, geom_point, labs
...
fig = (
    ggplot(ht, aes(x=ht.idx, y=ht.squared))
    + geom_point(aes(color=ht.even, shape=ht.threeven))
    + labs(color="Even", shape="Threeven")
)
...

Generating this legend:

For more information on the current behavior, see #12244 and #12207.

[danking]

Just wanted to say this is dope! Particularly considering this issue has been open since 2020.

[patrick-schultz]

Really nice work! This is a great change that I'm happy to merge now, but this suggests a number of follow-up improvements:

• we should plan to abstract out the legend handling code to apply uniformly to all geoms
• in the common case where an aesthetic is mapped to a field (as opposed to a compound expression), we should use the field name as the default legend group name. E.g. your second example with custom labels should be the default (but without capitalizing). This matches ggplot behavior.
• if multiple aesthetics are mapped to the same value, they should only generate one legend group. E.g. geom_point(aes(color=ht.even, shape=ht.even)) should generate one group with title "even", with values "no" a blue circle and "yes" a red square.

addressed

[patrick-schultz]

Sorry, could you also add an example to the docstring for labs? It isn't obvious that the keys need to be aesthetic names.

[danking]

Another follow up PR: update the ggplot tutorial to include the use of cross-product legends!

[happydangoe]

Hey everyone! Thank you so much for working on this. As the requestor of many ggplot features -- including this one -- I am so thankful for all your effort in making this a great graphics language :)