How to add a hline or vline to a plotly express legend in Python

Question

I'm trying to add my hline to the legend and label it as 'mean'. Is this possible and if so how.

Example

import numpy as np
import pandas as pd
import plotly.express as px

df = pd.DataFrame({'day': np.repeat(np.arange(1, 11, 1), 2),
                   'value': np.repeat([1, 2, 3, 4, 5], 4),
                   'var': list(['A', 'B']) * 10})

fig = px.bar(df,
             x='day',
             y='value',
             color='var')

fig.add_hline(y=df['value'].mean(), line_dash="dot")

fig.show()

Expected outcome

Answer 1

You can use the annotation_text parameter in fig.add_hline:

fig.add_hline(y=df['value'].mean(), line_dash="dot", annotation_text="mean")

That would add a text right above the dotted line:

However, the text will not be right under the "Var" section.
In order to do this, you will need to change it from add_hline to add_trace as it is not subject to the legend:

import numpy as np
import pandas as pd
import plotly.express as px
import plotly.graph_objects as go

df = pd.DataFrame({'day': np.repeat(np.arange(1, 11, 1), 2),
                   'value': np.repeat([1, 2, 3, 4, 5], 4),
                   'var': list(['A', 'B']) * 10})

fig = px.bar(df,
             x='day',
             y='value',
             color='var')

fig.add_trace(go.Scatter(
    x=df['day'],
    y=[df['value'].mean()] * len(df['day']),
    mode='lines',
    name='mean',
    line=dict(dash='dot', width=2, color='black'),
))

fig.show()

Which results in this figure: