Plotly: Plotly express area chart with a single color for fill and line

Question

Hi i try to figure out how to use just one color per country - filled color. The following example gives a mix of transparent and a darker color per country. i expect just one color. Thanks for help

import plotly
#import plotly.graph_objects as go
import plotly.express as px
df = px.data.gapminder()
fig = px.area(df, x="year", y="pop", color="country",
          groupnorm='fraction')
plotly.io.write_image(fig, file='areatest.png', format='png')

Answer 1

There's no px.area attribute that will give you a single color directly. But that should not stop you. Just use:

fig.for_each_trace(lambda trace: trace.update(fillcolor = trace.line.color))

... to get:

... instead of the default result which is:

Complete code:

import plotly
import plotly.express as px
df = px.data.gapminder()
df = df[df['continent']=='Americas']
fig = px.area(df, x="year", y="pop", color="country",
          groupnorm='fraction')
fig.for_each_trace(lambda trace: trace.update(fillcolor = trace.line.color))
fig.show()

Some details:

px.area will assign a color to each line associated with the color argument, and then apply a partially transparent version of that color to the corresponding area / fill. If you run fig.show you can check the color for the first category:

Scatter({
    'fillcolor': '#636efa',
    'groupnorm': 'fraction',
    'hovertemplate': 'country=Argentina<br>year=%{x}<br>pop=%{y}<extra></extra>',
    'legendgroup': 'Argentina',
    'line': {'color': '#636efa'},

Here you can only see that 'line': {'color': '#636efa'} and it can undoubtedly be a bit confusing that fillcolor is nowhere to be found . But if you use fig.full_figure_for_development, you can see that the fillcolor that is in fact passed to javascript is 'rgba(99, 110, 250, 0.5)':

Scatter({
    'connectgaps': False,
    'error_x': {'visible': False},
    'error_y': {'visible': False},
    'fill': 'tonexty',
    'fillcolor': 'rgba(99, 110, 250, 0.5)',

And that's exactly what you're overriding when you run:

fig.for_each_trace(lambda trace: trace.update(fillcolor = trace.line.color))