Support for Basemap

@nimisha-18, I was able to add a basemap by implementing a custom geom subtype that adds the basemap during plotnine.ggplot's draw phase:

from __future__ import annotations

import typing

import pandas as pd
from plotnine.geoms.geom import geom
import contextily as ctx

if typing.TYPE_CHECKING:
    from typing import Any

    import pandas as pd
    from matplotlib.axes import Axes

    from plotnine.coords.coord import coord
    from plotnine.iapi import panel_view


class geom_basemap(geom):
    DEFAULT_PARAMS = {
        "stat": "identity",
        "position": "identity",
        "na_rm": False,
        "crs": "EPSG:3857",
        "source": ctx.providers.OpenStreetMap.Mapnik,
    }

    def draw_panel(
        self,
        data: pd.DataFrame,
        panel_params: panel_view,
        coord: coord,
        ax: Axes,
        **params: Any,
    ):
        crs = params["crs"]
        source = params["source"]
        ctx.add_basemap(ax, crs=crs, source=source)

To use it in your example, you would just add it like any other layer:

plot3 = plot2 + geom_basemap(source=ctx.providers.OpenStreetMap.Mapnik)
plot3.show()

Aside, the geom_basemap concept as a geom, at least to me, doesn't fit well with the ideas outlines in The Grammar of Graphics. Im not sure what category a basemap falls within so calling this a geom is likely conceptually incorrect. Just something to think about.

Originally posted by @aaraney in https://github.com/has2k1/plotnine/discussions/850#discussioncomment-12436571

Start by reviewing the proposed custom geom_basemap implementation and how plotnine.ggplot handles its draw phase. Compare the contextily basemap example with the Grammar of Graphics concepts, then determine the appropriate API and category for basemap support. Done means an agreed design with implementation and coverage for the supported usage.