Plot curve with blending line colors with matplotlib/pyplot

Question

I want to start the curve with one color and progressively blend into another color until the end. The following function in my MCVE works, but surely, there has to be a better way I haven't found out about, yet?!

import numpy as np
import matplotlib.pyplot as plt

def colorlist(color1, color2, num):
    """Generate list of num colors blending from color1 to color2"""
    result = [np.array(color1), np.array(color2)]
    while len(result) < num:
        temp = [result[0]]
        for i in range(len(result)-1):
            temp.append(np.sqrt((result[i]**2+result[i+1]**2)/2))
            temp.append(result[i+1])
        result = temp
    indices = np.linspace(0, len(result)-1, num).round().astype(int)
    return [result[i] for i in indices]

x = np.linspace(0, 2*np.pi, 100)
y = np.sin(x)
colors = colorlist((1, 0, 0), (0, 0, 1), len(x))

for i in range(len(x)-1):
    xi = x[i:i+1+1]
    yi = y[i:i+1+1]
    ci = colors[i]
    plt.plot(xi, yi, color=ci, linestyle='solid', linewidth='10')

plt.show()

ImportanceOfBeingErnest · Accepted Answer · 2017-12-17 19:24:01Z

11

Not sure what "better way" refers to. A solution with less code, which would draw faster is the use of a LineCollection together with a colormap.

A colormap can be defined by two colors and any colors in between are automatically interpolated.

cmap = matplotlib.colors.LinearSegmentedColormap.from_list("", [(1, 0, 0), (0, 0, 1)])

A LineCollection can be used to plot a lot of lines at once. Being a ScalarMappable it can use a colormap to colorize each line differently according to some array - in this case one may just use the x values for that purpose.

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.collections import LineCollection
from matplotlib.colors import LinearSegmentedColormap

x = np.linspace(0, 2*np.pi, 100)
y = np.sin(x)

cmap = LinearSegmentedColormap.from_list("", [(1, 0, 0), (0, 0, 1)])

points = np.array([x, y]).T.reshape(-1,1,2)
segments = np.concatenate([points[:-1],points[1:]], axis=1)

lc = LineCollection(segments, cmap=cmap, linewidth=10)
lc.set_array(x)
plt.gca().add_collection(lc)
plt.gca().autoscale()
plt.show()

The drawback of this solution as can be see in the picture is that the individual lines are not well connected.

So to circumvent this, one may plot those points overlapping, using

segments = np.concatenate([points[:-2],points[1:-1], points[2:]], axis=1)

In the above the color is linearly interpolated between the two given colors. The plot therefore looks different than the one from the question using some custom interpolation.

To obtain the same colors as in the question, you may use the same function to create the colors used in the colormap for the LineCollection. If the aim is to simplify this function you may directly calculate the values as the square root of the color difference in the channels.

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.collections import LineCollection
from matplotlib.colors import LinearSegmentedColormap

x = np.linspace(0, 2*np.pi, 100)
y = np.sin(x)

def colorlist2(c1, c2, num):
    l = np.linspace(0,1,num)
    a = np.abs(np.array(c1)-np.array(c2))
    m = np.min([c1,c2], axis=0)
    s  = np.sign(np.array(c2)-np.array(c1)).astype(int)
    s[s==0] =1
    r = np.sqrt(np.c_[(l*a[0]+m[0])[::s[0]],(l*a[1]+m[1])[::s[1]],(l*a[2]+m[2])[::s[2]]])
    return r

cmap = LinearSegmentedColormap.from_list("", colorlist2((1, 0, 0), (0, 0, 1),100))

points = np.array([x, y]).T.reshape(-1,1,2)
segments = np.concatenate([points[:-2],points[1:-1], points[2:]], axis=1)

lc = LineCollection(segments, cmap=cmap, linewidth=10)
lc.set_array(x)
plt.gca().add_collection(lc)
plt.gca().autoscale()
plt.show()

edited Dec 17, 2017 at 19:24

answered Dec 17, 2017 at 14:30

ImportanceOfBeingErnest

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6 Comments

Tim Stack Over a year ago

How does one change this to work on Y-axis values instead?

Not_a_programmer Over a year ago

@TimStack You could use something like colors = [cmap(k) for k in y[:-1]] and lc = LineCollection(segments, colors=colors, linewidth=10)

Tim Stack Over a year ago

@Not_a_programmer I am afraid that only changes the colour hues

Not_a_programmer Over a year ago

@TimStack Apparently, I'm missing the point then. I thought your question was about changing the line's color depending on its y value. That's what my code intends to do. However, I forgot to mention that y values must be normalized, as shown in this post.

Tim Stack Over a year ago

@Not_a_programmer That's correct. However, with the code above and your amendments, the hues of the last figure in this post are changed, and are still dependent on the X-axis

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Not_a_programmer · Accepted Answer · 2022-05-26 15:01:23Z

0

In response to a comment above: If you want to change the color depending on the y value, you can use the following code:

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.collections import LineCollection
from matplotlib.colors import LinearSegmentedColormap

x = np.linspace(0, 2 * np.pi, 100)
y = np.sin(x)
ynorm = (y - y.min()) / (y.max() - y.min())


def colorlist2(c1, c2, num):
    l = np.linspace(0, 1, num)
    a = np.abs(np.array(c1) - np.array(c2))
    m = np.min([c1, c2], axis=0)
    s = np.sign(np.array(c2) - np.array(c1)).astype(int)
    s[s == 0] = 1
    r = np.sqrt(np.c_[(l * a[0] + m[0])[::s[0]],
                      (l * a[1] + m[1])[::s[1]], (l * a[2] + m[2])[::s[2]]])
    return r


cmap = LinearSegmentedColormap.from_list(
    "", colorlist2((1, 0, 0), (0, 0, 1), 100))
colors = [cmap(k) for k in ynorm[:-1]]

points = np.array([x, y]).T.reshape(-1, 1, 2)
segments = np.concatenate([points[:-2], points[1:-1], points[2:]], axis=1)

lc = LineCollection(segments, colors=colors, linewidth=10)
lc.set_array(x)
plt.gca().add_collection(lc)
plt.gca().autoscale()
plt.show()

This will output this graph:

Graph with color depending on y value

answered May 26, 2022 at 15:01

Not_a_programmer

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4 Comments

Tim Stack Over a year ago

I ran the code in your post, but the resulting figure still has colours dependant on the x-axis.. odd!

Not_a_programmer Over a year ago

Are you sure that you used the y values as inputs for the colors instead of the x values? That's the only thing I can think about

Tim Stack Over a year ago

I have simply copied your code without make any alterations

Not_a_programmer Over a year ago

This is very strange as the colormap of your graph is also different. I'm afraid I'm out of ideas about what could've gone wrong. Sorry, I can't help you.

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