how to convert continuous colorscale into discrete colorscale?

Question

I'd like to use two colors red and blue but with different concentration like below. I'd like to convert this continuous color scale into discreate color scale with 10 discreate colors.

https://plotly.com/python/colorscales/#reversing-a-builtin-color-scale

If I print the continuous colorscale, it has only 2 element in the list like below. Now How can I get 10 discreate colors between red and blue with different concentrations. Thanks

    colors=px.colors.sequential.Bluered_r
    
    print(colors)

    ['rgb(255,0,0)', 'rgb(0,0,255)']

Konstantin Z · Answer 1 · 2022-09-06T18:24:42.760

UPD

There is a simpler way with sample_colors from this answer

from plotly.express.colors import sample_colorscale
from sklearn.preprocessing import minmax_scale

colors_ = [1,5,6,7,8] 
discrete_colors = sample_colorscale('Bluered', minmax_scale(colors_))

# colors_ = 5 numbers you are trying to depict with the colorscale
# discrete_colors - list of 5 rgb-coded colors from *Bluered* colorscale
# minmax_scale is used because *sample colors* can only deal with floats from [0,1]

Old answer

As far as I know, plotly doesn't have an explicit function for that. For a red-blue scale a simple np.linspace-based implementation should work.

import numpy as np
def n_discrete_rgb_colors(color1: str, color2: str, n_colors: int) -> list:
    color1_ = [int(i) for i in color1[4:-1].split(",")]
    color2_ = [int(i) for i in color2[4:-1].split(",")]
    colors_ = np.linspace(start = color1_, stop = color2_, num = n_colors)
    colors = [str(f"rgb{int(i[0]),int(i[1]),int(i[2])}") for i in colors_]
    return colors

color1, color2 = ['rgb(255,0,0)', 'rgb(0,0,255)']
# color1, color2 = px.colors.sequential.Bluered_r
n_discrete_rgb_colors(color1, color2, 10)

Output

['rgb(255, 0, 0)',
 'rgb(226, 0, 28)',
 'rgb(198, 0, 56)',
 'rgb(170, 0, 85)',
 'rgb(141, 0, 113)',
 'rgb(113, 0, 141)',
 'rgb(85, 0, 170)',
 'rgb(56, 0, 198)',
 'rgb(28, 0, 226)',
 'rgb(0, 0, 255)']

Logic of the code is the following:

take two strings for edge colors in 'rgb(x,y,z)' format,
convert them into [x,y,z] lists,
build a linspace,
return this linspace with appropriate formatting

Thanks Konstantin. It works too. I hope I can accept two answers. I appreciate it — roudan, Mar 22 '22 at 20:55

score 1 · Accepted Answer · answered Mar 22 '22 at 19:41

really what you are looking at is https://plotly.com/python/colorscales/#constructing-a-discrete-or-discontinuous-color-scale
below code shows how to use a list comprehension to construct a discontinuous color scale from a continuous color scale

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

df = pd.DataFrame({c:np.linspace(1,10,100) for c in list("xyc")})

# https://plotly.com/python/colorscales/#constructing-a-discrete-or-discontinuous-color-scale
cmap = [
    (r, c)
    for r, c in zip(
        np.repeat(np.linspace(0, 1, len(px.colors.sequential.RdBu)+1), 2)[1:],
        np.repeat(px.colors.sequential.RdBu,2),
    )
]

px.bar(df, x="x", y="y", color="c", color_continuous_scale=cmap)

how to convert continuous colorscale into discrete colorscale?

2 Answers2