Feature: Support for Ibis expressions

Hi! 👋🏻

I'm the lead developer of the Ibis project.

I recently did a write up of some analysis of Ibis's CI data using Ibis and plotnine.

I think plotnine is just the bee's knees, and I think ibis and plotnine could be even better together than they are right now.

What would it take to support ibis expressions? I am happy to take this all the way through myself, PR-ing to this repo as well as make any fixes to ibis that might be needed, but I want to check in to make sure this work would have a chance at getting in!

Here's an example from the above notebook where it would be great if I could pass in my ibis objects without having to first turn them into pandas DataFrames:

    t = ... # an ibis table expression
    df = t.execute()
    ggplot(
        df.loc[df.entity == "job"].reset_index(drop=True),
        aes(x="started_date", y="duration", color="factor(improvements)"),
    )
    ... # more plotting code follows

Instead, I'd like to write the following:

    t = ...  # an ibis table expression
    # _ is a placeholder that means "the child table", here it's referring to the t.filter(...) call
    ggplot(
        t.filter(_.entity == "job"),
        aes(x=_.started_date, y=_.duration, color="factor(improvements)"),
    )
    ... # more plotting code follows

Begin at the ggplot entry point and compare the existing pandas DataFrame example with the proposed Ibis expression usage. Determine the scope needed for filtering and aesthetic expressions without calling execute(); done means the supplied Ibis table expressions work directly in the plotting example.