Why does Seaborn lineplot "size" argument end up as legend artist?

Question

In a simple lineplot from Seaborn sample data, adding a "size" argument to control linewidth automatically adds an artist/handle to the legend generated from the "label" argument.

import seaborn as sns
from matplotlib import pyplot as plt

df = sns.load_dataset('geyser')

fig, ax = plt.subplots()

sns.lineplot(
    x=df.waiting,
    y=df.duration,
    label='Label',
    size=3,
    ax=ax
)
plt.show()

What is the reason for this behavior, and what can be done to prevent it?

Answer 1

The size is used to group, and will have different size lines based on a category.

For example you can have different size based on the kind column:

import seaborn as sns
from matplotlib import pyplot as plt

df = sns.load_dataset('geyser')

fig, ax = plt.subplots()

sns.lineplot(
    x=df.waiting,
    y=df.duration,
    label='Label',
    size = df['kind'],
    ax=ax
)
plt.show()

Not sure what it's doing as a number though. You use linewidth to set the line size though:

import seaborn as sns
from matplotlib import pyplot as plt

df = sns.load_dataset('geyser')

fig, ax = plt.subplots()

sns.lineplot(
    x=df.waiting,
    y=df.duration,
    label='Label',
    linewidth = 3,
    ax=ax
)
plt.show()

Answer 2

Use the linewidth parameter to set the width of the line. The size parameter does something else. Check out the examples in the docs to see how to use it. The image below gives a good impression, and also makes it clear why the parameter results in a legend entry.