Support for Basemap

[has2k1]

@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 #850 (reply in thread)

[has2k1]

@aaraney You are right about geom_basemap. A better solution may be adding parameters to geom_map that turnon basemap. Something like.

geom_map(
    basemap={
        "source": ctx.providers.OpenStreetMap.Mapnik,
         "crs": "EPSG:3857", # if None, the default is a value from the geodataframe
        ...
    }
)

I hope to work on the mapping projections (supported by cartopy) later this year and I will be in a better position to think about how to add basemap support.

[aaraney]

@has2k1, the previous comment by @slapes7 looks like its part of an ongoing malware attack: https://any.run/report/a96b7342c5dc45bcb79b0cb7839d9c89461c4faec5ee3278fc56729ebd872262/926ed16a-6155-4cb1-a696-53125217e4ff. Can you remove it / report it?

[has2k1]

@aaraney I think github has detected it and others and removed them. Without your note, I was confused about the notifications.

[atchley-sha]

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.

For what it's worth, the ggspatial R package adds a basemap via annotation_map_tile(), so calling a basemap an annotation seems reasonable (maybe annotation_basemap or something like that).
Having a basemap as an option in geom_map works too, though it might be easy to run into weird behavior if you have multiple geom_map layers (but depending on how it's implemented that may not be a problem at all).

ggspatial has some other nice features for mapmaking such as adding scales and north arrows too. I'm not sure how high of a priority those would be as a feature request; maybe it even makes sense to follow the R approach and have a separate package that mirrors ggspatial in the way that plotnine mirrors ggplot2.