Legend title does not appear in dx plot by

[jnumainville]

In px, when there is a "plot by" equivalent, the legend title is set. This is not being set in dx.

import plotly.express as px
import deephaven.plot.express as dx

df = px.data.tips()

dx_bar = dx.bar(df, x="size", y=["tip", "total_bill"])
px_bar = px.bar(df, x="size", y=["tip", "total_bill"])

This is a minor bug on it's own, but it could be worth considering if we want to also expose a new arg legend_title in all plots when fixing this, as it would be mostly trivial to do in addition to the minor work necessary here.