[Feature request] Trace-specific color sequences in Plotly Express via templates

[antonymilne]

Hello! I'm a huge fan of plotly and especially plotly.express, but there's something that's bothers me a lot...

Problem

There's no way to make a template use different color sequences for different trace types in plotly express. Currently it always uses template.layout.colorway (unless explicitly overridden) and ignores per-trace defaults in template.data.<trace_type>.

Example

I'll use histogram and scatter here as an example but it applies very generally.

Let's say I want to use colors ["orange", "purple", "grey"] for a histogram. These are different from the default qualitative palette template.layout.colorway. Plotly enables you to give trace-specific colors like the below but it doesn't work: Plotly express ignores template.data.histogram and uses the same template.layout.colorway palette for both plots.

import plotly.express as px
import plotly.io as pio
import plotly.graph_objects as go

histogram_colors = ["orange", "purple", "grey"]

# Set histogram-specific colors in the template
histogram_template = go.layout.Template(
    data={"histogram": [{"marker": {"color": histogram_colors}}]}
)
template = pio.templates.merge_templates("plotly", histogram_template)

df = px.data.iris()

# Expectation: scatter uses global layout.colorway; histogram uses ["orange","purple","grey"]
# Actual behaviour: both use global layout.colorway
fig1 = px.scatter(df, x="sepal_length", y="petal_length", color="species", template=template)
fig2 = px.histogram(df, color="species", template=template)

Current workarounds

• supply color_discrete_sequence or color_discrete_map to each px call
• use multiple templates with different layout.colorway and apply a different template to each chart

Why this matters

In reality I don't actually want to completely change colors between histogram and scatter. Actually what I'd like to do is apply the same palette but desaturated to the histogram. Seaborn supports this e.g. barplot(saturation=0.75).

I would like to be able to achieve this with a single template and without manually supplying arguments on per-figure basis.

Proposed solution

Extend plotly express's discrete color resolution to look at trace-specific colors in preference to template.layout.colorway. A single Plotly template already has space to supply a different color palette on each trace by using template.data, and Plotly express does look in template.data but only for a very limited number of things, e.g. it looks at template.data.scatter for line_dash_sequence. But color_discrete_sequence is always taken from template.layout.colorway rather than looking at template.data.

Instead, plotly could take color_discrete_sequence from the relevant template.data trace and then, if that's not defined, fall back on the current mechanism to default to template.layout.colorway.

Alternative solutions

• a new saturation argument in plotly schema, like there's already opacity. This wouldn't allow arbitrary changes between different trace types but just to desaturate
• a new saturation argument in plotly.express that would automatically apply the color transformation, like in seaborn. No need to modify the plotly figure schema
• ability to define different color sequences for different trace types in plotly express. plotly.express.defaults currently defines a single color_discrete_sequence, but there could be a new color_discrete_sequence_by_trace = {"histogram": ..., ...} or similar

[antonymilne]

@huong-li-nguyen pointed me to this comment originally posted by @nicolaskruchten in plotly_express issue #70:

In a template, we have color scales and sequences in a few places in layout such as layout.colorway, layout.piecolorway, layout.sunburstcolorway, layout.colorscale.sequential, layout.colorscale.sequentialminus, layout.colorscale.diverging but also on a per-trace-type basis: a template can specify one colorscale for scatter and another for heatmaps, and it can also specify that the first scatter is one color and the second is another, at which point I think that scatter traces will end up alternating between the two colors. So... yeah, there are a lot of ways to specify colors in a template.

All that said, px could just ignore most of that complexity and try to read from template.layout.colorway and template.layout.colorscale.sequential and call it a day :)

So it would seem that there are already places in layout to specify multiple colorways for five trace types (pie, funnelarea, sunburst, icicle, treemap). However, these do not exist for most trace types and are ignored by plotly express anyway.

In short I would like to ask whether the decision to "ignore most of that complexity" could be reconsidered to allow per-trace-type color scales. Or for there to be some other easy way to tweak a single color palette (e.g. by its saturation).

[emilykl]

@antonymilne Thanks for the suggestion!

I agree that having a way to specify different default colorways for different trace types would be a useful feature.

There is some complexity here with adding colorway into the template under data, though. The template follows a very specific format which mirrors the available Plotly.js attributes for layout and traces. Attributes passed under data are applied directly to the corresponding traces. Currently, Plotly.js traces have no colorway attribute, which is why passing colorway it in the template under data has no effect. The reason traces don't have a colorway is because individual Plotly.js traces use either a single solid color, or a continuous colorscale based on the data. Plots with discrete color scales in Plotly Express are represented as multiple single-color traces in Plotly.js. The colorway attribute is therefore a property of the entire plot, which tells Plotly.js which color to use each time a new trace is added.

Off the top of my head I can think of a few possible ways of implementing this feature:

• In Plotly.py: Modify px.defaults to support per-trace-type color_discrete_sequence values. I'm not immediately sure what the best API would be; perhaps we could allow color_discrete_sequence to be either a color sequence, or a dictionary mapping trace names to color sequences.

• In Plotly.js: Add additional <tracename>colorway layout attributes for every trace type which uses colorways. I don't love this option as it would add a lot of top-level attributes to the layout, but it would provide the requested functionality. These <tracename>colorway attributes could then be given values in the template.

• In Plotly.js: Add a new layout attribute, something like tracecolorways, which accepts a dictionary mapping trace names to colorways. Solves the problem of adding lots of top-level attributes to layout but would have to think some more about the pros and cons of this approach. Also would be redundant with existing <tracename>colorway.

Open to other suggestions and we're happy to discuss further if you'd like to make an API proposal and/or work on a PR.