Plotly: How to change the size of the lines displayed in the legend?

Question

I am having a really tough time to make legends bigger in plotly. I've exhausted the docs, forums, github issues and have found nothing. Starting to think plotly isn't that great of software.

I created this graph:

And I want to make the lines bigger in the legend for Trial 1, Trial 2, and Trial 3. I can make the font bigger using their api but I see no reference to the legend lines and now wondering whether it is possibe.

Here's some Code:

fig.update_layout(
    title_text="Dipole Moment X (0 V/nm)",
    title_font=dict(size=44, family='Arial'),
    template='simple_white',
    xaxis_tickformat = 'i',
    bargap=0.2, # gap between bars of adjacent location coordinates,
    legend=dict(
        orientation="h",
        yanchor="bottom",
        y=1.02,
        xanchor="right",
        x=1,  
        font = dict(family = "Arial", size = 60),
        bordercolor="LightSteelBlue",
        borderwidth=2,
        itemsizing='trace'
    ),
    legend_title = dict(font = dict(family = "Arial", size = 60)),
)

Played around with the itemsizing and got nothing either. Anyone have any idea on how to accomplish this?

UPDATE:

Based on the answer below I was able to get the line thicker but there is a limit. And attached is the limit of thickness I believe it to be (don't know the exact size)

Answer 1

The answer:

Depending on how you've set up your figure, you can use:

fig.update_layout(legend=dict(itemsizing='constant'))

Or:

fig.update_layout(legend=dict(itemsizing='trace'))
fig.update_traces(line=dict(width=12))

The details:

You seem to have opted for very thin lines when setting up the figure. You can "detach" the width of the lines in the legend from the width of the trace lines using:

fig.update_layout(legend=dict(itemsizing='constant'))

Since you haven't produced a runnable code snippet, I'll show the effect using data from px.data.gapminder().

Plot 1 - Without fig.update_layout(legend=dict(itemsizing='constant'))

Plot 2 - With fig.update_layout(legend=dict(itemsizing='constant'))

Your third option is to set fig.update_layout(legend=dict(itemsizing='trace')) and then increase the line width for the traces and the legend using, for example, fig.update_traces(line=dict(width=12)):

Plot 3 - With fig.update_layout(legend=dict(itemsizing='trace'))

Complete code with all options available

import plotly.express as px

df = px.data.gapminder().query("continent=='Oceania'")
fig = px.line(df, x="year", y="lifeExp", color='country')

fig.update_layout(showlegend=True)
fig.update_layout(legend = dict(bgcolor = 'yellow'))

#fig.update_layout(legend=dict(itemsizing='constant'))
fig.update_layout(legend=dict(itemsizing='trace'))
fig.update_traces(line=dict(width=12))

fig.show()