[matplotlib]How to Plot a Gradient Color Line[colormap]

This article shows how to plot a gradient color line in matplotlib.

The following article describes how to change the color of a graph element.

How to Specify a Color

About colormap

To plot a gradient color line, it is useful to use a colormap.

I show the colormap.

The following five types of color maps are available.

• Sequential
• Diverging
• Cyclic
• Qualitative
• Miscellaneous

I show each of these colormaps.

Sequential

"Sequential" is a color map that continuously changes hue and saturation of colors.

It is suitable for graphs where you want to express order.

Diverging

"Diverging" is a color map that changes hue in the center.

It is suitable when the median has important data.

Cyclic

"Cyclic" is a colormap that matches the brightness of the data at the edges.

For example, it is suitable for representing data such as wind direction and angle.

Qualitative

"Qualotative" is a color map with disparate hues.

For example, it is suitable for pie chart.

Miscellaneous

Other colormaps.

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Gradient color for scatter plot

I show how to apply a colormap to a scatter plot.

Matplotlib provides methods for drawing a scatter plot.

The way to use of "matplotlib.pyplot.scatter" is described in matplotlib documentation.

A colormap can be applied by specifying the argument "cmap=".

This is the example code.

import matplotlib.pyplot as plt
import matplotlib.cm as cm
import numpy as np

x = np.random.rand(100)
y = np.random.rand(100)
value = np.random.rand(100)

fig = plt.figure(figsize = (6,4), facecolor='lightblue')
plt.xlabel('X')
plt.ylabel('Y')

plt.scatter(x, y, s=50, c=value, cmap='Blues')
 
plt.colorbar(ticks=np.arange(0, 1, 0.1))

plt.show()

The above code generates the following graph.

plt.scatter(x, y, s=50, c=value, cmap='Blues')

The colormap "Blues" is passed to argument "cmap=".

plt.colorbar(ticks=np.arange(0, 1, 0.1))

Color bar is displayed.

Gradient color for line graph

I show how to apply a colormap to a line graph.

Matplotlib provides methods for drawing a line graph.

The way to use of "matplotlib.pyplot.plot" is described in matplotlib documentation.

Unlike scatter plots, color maps cannot be specified as arguments.

To make a line graph color gradient, specify one color at a time as the argument.

This is the example code.

import matplotlib.pyplot as plt
import matplotlib.cm as cm
import numpy as np

num = 10

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

fig = plt.figure(figsize = (6,4), facecolor='lightblue')
plt.xlabel('X')
plt.ylabel('Y')

for i in range(num):
    y = np.sin(x) + i
    plt.plot(x, y, linestyle='solid', label=str(i), color=cm.Blues(i/num))

plt.legend(loc=(1,0.29))

plt.show()

The above code generates the following graph.

> plt.plot(x, y, linestyle='solid', label=str(i), color=cm.Blues(i/num))

The color of colormap "Blues" is passed to the argument "color=" one by one.

> plt.legend(loc=(1,0.29))

Legend is added.

The following article explains how to add legend.

How to Add Legend

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Gradient color for bar plot

I show how to apply a colormap to a bar plot.

Matplotlib provides methods for drawing a bar plot.

The way to use of "matplotlib.pyplot.bar" is described in matplotlib documentation.

Unlike scatter plots, color maps cannot be specified as arguments.

To make a bar color gradient, specify one color at a time as the argument.

This is the example code.

import matplotlib.pyplot as plt
import matplotlib.cm as cm
import numpy as np

num = 10

fig = plt.figure(figsize = (6,4), facecolor='lightblue')
plt.xlabel('X')
plt.ylabel('Y')
plt.xticks(np.arange(0, num, 1))

for x in range(num):
    y = 1
    plt.bar(x, y, color=cm.Blues(x/num), align='center')

plt.show()

The above code generates the following graph.

> plt.bar(x, y, color=cm.Blues(x/num), align='center')

The color of colormap "Blues" is passed to the argument "color=" one by one.

[Supplement]Gradient color without using colormap

I show how to use gradient color without using colormap.

By passing an RGB tuple as the argument "color=", you can make the graph color gradient.

This is the example code.

import matplotlib.pyplot as plt
import numpy as np

num = 10

fig = plt.figure(figsize = (6,4), facecolor='lightblue')
plt.xlabel('X')
plt.ylabel('Y')
plt.xticks(np.arange(0, num, 1))

for x in range(num):
    y = 1
    plt.bar(x, y, align='center', color=(0,0,x/num))

plt.show()

The above code generates the following graph.

> plt.bar(x, y, align='center', color=(0,0,x/num))

The value of Blue in the RGB tuple is changed.

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