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:111: PlotnineWarning: geom_path: Each group consist of only one observation. Do you need to adjust the group aesthetic?

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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')
)

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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')
)

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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')
)

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Credit: github user [@datavistics](https://github.com/datavistics) (derek) whose encounter with this issue motivated this example.