When the automatic groups are not sufficient

import pandas as pd
from plotnine import ggplot, aes, geom_col, geom_line, labs, scale_color_hue

Some data to plot

df = pd.DataFrame(
    {
        "letter": ["Alpha", "Beta", "Delta", "Gamma"],
        "pos": [1, 2, 3, 4],
        "num_of_letters": [5, 4, 5, 5],
    }
)

df

	letter	pos	num_of_letters
0	Alpha	1	5
1	Beta	2	4
2	Delta	3	5
3	Gamma	4	5

(
    ggplot(df)
    + geom_col(aes(x="letter", y="pos"))
    + geom_line(aes(x="letter", y="num_of_letters"))
    + labs(title="Greek Letter Analysis")
)

/Users/hassan/scm/python/plotnine/plotnine/geoms/geom_path.py:113: PlotnineWarning: geom_path: Each group consist of only one observation. Do you need to adjust the group aesthetic?

We get a plot with a warning and no line(s). This is not what we expected.

The issue is we have 4 groups (Alpha, Beta, …) and each of those groups has a single point. This is a case where the automatic grouping is not sufficient (or just not what you expect). The solution is to manually set the group for geom_line so that all points belong to one group.

(
    ggplot(df)
    + geom_col(aes(x="letter", y="pos"))
    + geom_line(aes(x="letter", y="num_of_letters"), group=1)
    + labs(title="Greek Letter Analysis")
)

That looks like it.

To understand why the behaviour behind the warning is not wrong, let us try a dataframe with 2 points per group.

df2 = pd.DataFrame(
    {
        "letter": ["Alpha", "Beta", "Delta", "Gamma"] * 2,
        "pos": [1, 2, 3, 4] * 2,
        "num_of_letters": [5, 4, 5, 5] * 2,
    }
)

df2.loc[4:, "num_of_letters"] += 0.8

(
    ggplot(df2)
    + geom_col(aes(x="letter", y="pos"))
    + geom_line(aes(x="letter", y="num_of_letters"))
    + labs(title="Greek Letter Analysis")
)

We get no warning and we get lines.

We can add some color to such a plot.

(
    ggplot(df2)
    + geom_col(aes(x="letter", y="pos", fill="letter"))
    + geom_line(aes(x="letter", y="num_of_letters", color="letter"), size=1)
    + scale_color_hue(l=0.45)  # some contrast to make the lines stick out
    + labs(title="Greek Letter Analysis")
)

Credit: github user @datavistics (derek) whose encounter with this issue motivated this example.