2  Examples from the ggplot2 documentation

The following examples are adapted from the ggplot2 book’s coord chapter.

Caution

All of the examples on this page require the coord-polar branch of iangow/plotnine. The standard version of plotnine does not include the coord_polar() and coord_radial() functions used here.

The source code for this repository can be found here and includes a pyproject.toml file that can be used with uv to install everything needed to run all the examples here.

import numpy as np
import polars as pl
from plotnine import (
    ggplot, aes,
    geom_bar, geom_path, geom_point,
    coord_polar,
    scale_x_discrete, scale_y_continuous,
    theme,
)
from plotnine.data import mtcars

2.1 Bar → Pie → Bullseye (mtcars)

A stacked bar chart becomes a pie chart when theta = "y" maps height (count) to angle, and a bullseye chart when the default theta = "x" maps the discrete x position to angle instead.

mtcars_pl = (
    pl.from_pandas(mtcars)
    .with_columns(
        pl.lit("1").alias("x"),
        pl.col("cyl").cast(pl.Utf8),
    )
)

(
    ggplot(mtcars_pl, aes(x="x", fill="cyl"))
    + geom_bar(width=1)
    + scale_x_discrete(expand=(0, 0))
    + scale_y_continuous(expand=(0, 0))
    + theme(legend_position="none", figure_size=(4, 4))
)

Figure 2.1: Stacked bar chart

(
    ggplot(mtcars_pl, aes(x="x", fill="cyl"))
    + geom_bar(width=1)
    + coord_polar(theta="y")
    + scale_x_discrete(expand=(0, 0))
    + scale_y_continuous(expand=(0, 0))
    + theme(legend_position="none", figure_size=(4, 4))
)

Figure 2.2: Pie chart: coord_polar(theta=‘y’)

(
    ggplot(mtcars_pl, aes(x="x", fill="cyl"))
    + geom_bar(width=1)
    + coord_polar()
    + scale_x_discrete(expand=(0, 0))
    + scale_y_continuous(expand=(0, 0))
    + theme(legend_position="none", figure_size=(4, 4))
)

Figure 2.3: Bullseye chart: coord_polar()

2.2 Polar path (spiral)

In ggplot2 a straight line in Cartesian space becomes a spiral in polar coordinates. theta = "x" maps the x-column (here: angle in radians) to the angular axis; y maps to radius.

n = 100
spiral = pl.DataFrame({
    "r":     np.linspace(0, 1, n),
    "theta": np.linspace(0, 3 / 2 * np.pi, n),
})

(
    ggplot(spiral, aes(x="theta", y="r"))
    + geom_path()
    + geom_point(size=2, color="red")
    + coord_polar()
    + theme(figure_size=(5, 5))
)

Figure 2.4: A Cartesian line becomes a spiral under coord_polar()