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Matplotlib Scatter

Creating Scatter Plots

With Pyplot, you can use the scatter() function to draw a scatter plot.

The scatter() function plots one dot for each observation. It needs two arrays of the same length, one for the values of the x-axis, and one for values on the y-axis:

Example

A simple scatter plot:

import matplotlib.pyplot as plt
import numpy as np

x = np.array([5,7,8,7,2,17,2,9,4,11,12,9,6])
y = np.array([99,86,87,88,111,86,103,87,94,78,77,85,86])

plt.scatter(x, y)
plt.show()

Result:

The observation in the example above is the result of 13 cars passing by.

The X-axis shows how old the car is.

The Y-axis shows the speed of the car when it passes.

Are there any relationships between the observations?

It seems that the newer the car, the faster it drives, but that could be a coincidence, after all we only registered 13 cars.

Compare Plots

In the example above, there seems to be a relationship between speed and age, but what if we plot the observations from another day as well? Will the scatter plot tell us something else?

Example

Draw two plots on the same figure:

import matplotlib.pyplot as plt
import numpy as np

#day one, the age and speed of 13 cars:
x = np.array([5,7,8,7,2,17,2,9,4,11,12,9,6])
y = np.array([99,86,87,88,111,86,103,87,94,78,77,85,86])
plt.scatter(x, y)

#day two, the age and speed of 15 cars:
x = np.array([2,2,8,1,15,8,12,9,7,3,11,4,7,14,12])
y = np.array([100,105,84,105,90,99,90,95,94,100,79,112,91,80,85])
plt.scatter(x, y)

plt.show()

Result:

Note: The two plots are plotted with two different colors, by default blue and orange, you will learn how to change colors later in this chapter.

By comparing the two plots, I think it is safe to say that they both gives us the same conclusion: the newer the car, the faster it drives.

Colors

You can set your own color for each scatter plot with the color or the c argument:

Example

Set your own color of the markers:

import matplotlib.pyplot as plt
import numpy as np

x = np.array([5,7,8,7,2,17,2,9,4,11,12,9,6])
y = np.array([99,86,87,88,111,86,103,87,94,78,77,85,86])
plt.scatter(x, y, color = 'hotpink')

x = np.array([2,2,8,1,15,8,12,9,7,3,11,4,7,14,12])
y = np.array([100,105,84,105,90,99,90,95,94,100,79,112,91,80,85])
plt.scatter(x, y, color = '#88c999')

plt.show()

Result:

Color Each Dot

You can even set a specific color for each dot by using an array of colors as value for the c argument:

Note: You cannot use the color argument for this, only the c argument.

Example

Set your own color of the markers:

import matplotlib.pyplot as plt
import numpy as np

x = np.array([5,7,8,7,2,17,2,9,4,11,12,9,6])
y = np.array([99,86,87,88,111,86,103,87,94,78,77,85,86])
colors = np.array(["red","green","blue","yellow","pink","black","orange","purple","beige","brown","gray","cyan","magenta"])

plt.scatter(x, y, c=colors)

plt.show()

Result:

ColorMap

The Matplotlib module has a number of available colormaps.

A colormap is like a list of colors, where each color has a value that ranges from 0 to 100.

Here is an example of a colormap:

This colormap is called 'viridis' and as you can see it ranges from 0, which is a purple color, up to 100, which is a yellow color.

How to Use the ColorMap

You can specify the colormap with the keyword argument cmap with the value of the colormap, in this case 'viridis' which is one of the built-in colormaps available in Matplotlib.

In addition you have to create an array with values (from 0 to 100), one value for each point in the scatter plot:

Example

Create a color array, and specify a colormap in the scatter plot:

import matplotlib.pyplot as plt
import numpy as np

x = np.array([5,7,8,7,2,17,2,9,4,11,12,9,6])
y = np.array([99,86,87,88,111,86,103,87,94,78,77,85,86])
colors = np.array([0, 10, 20, 30, 40, 45, 50, 55, 60, 70, 80, 90, 100])

plt.scatter(x, y, c=colors, cmap='viridis')

plt.show()

Result:

You can include the colormap in the drawing by including the plt.colorbar() statement:

Example

Include the actual colormap:

import matplotlib.pyplot as plt
import numpy as np

x = np.array([5,7,8,7,2,17,2,9,4,11,12,9,6])
y = np.array([99,86,87,88,111,86,103,87,94,78,77,85,86])
colors = np.array([0, 10, 20, 30, 40, 45, 50, 55, 60, 70, 80, 90, 100])

plt.scatter(x, y, c=colors, cmap='viridis')

plt.colorbar()

plt.show()

Result:

Available ColorMaps

You can choose any of the built-in colormaps:

Name   Reverse
Accent   Accent_r
Blues   Blues_r
BrBG   BrBG_r
BuGn   BuGn_r
BuPu   BuPu_r
CMRmap   CMRmap_r
Dark2   Dark2_r
GnBu   GnBu_r
Greens   Greens_r
Greys   Greys_r
OrRd   OrRd_r
Oranges   Oranges_r
PRGn   PRGn_r
Paired   Paired_r
Pastel1   Pastel1_r
Pastel2   Pastel2_r
PiYG   PiYG_r
PuBu   PuBu_r
PuBuGn   PuBuGn_r
PuOr   PuOr_r
PuRd   PuRd_r
Purples   Purples_r
RdBu   RdBu_r
RdGy   RdGy_r
RdPu   RdPu_r
RdYlBu   RdYlBu_r
RdYlGn   RdYlGn_r
Reds   Reds_r
Set1   Set1_r
Set2   Set2_r
Set3   Set3_r
Spectral   Spectral_r
Wistia   Wistia_r
YlGn   YlGn_r
YlGnBu   YlGnBu_r
YlOrBr   YlOrBr_r
YlOrRd   YlOrRd_r
afmhot   afmhot_r
autumn   autumn_r
binary   binary_r
bone   bone_r
brg   brg_r
bwr   bwr_r
cividis   cividis_r
cool   cool_r
coolwarm   coolwarm_r
copper   copper_r
cubehelix   cubehelix_r
flag   flag_r
gist_earth   gist_earth_r
gist_gray   gist_gray_r
gist_heat   gist_heat_r
gist_ncar   gist_ncar_r
gist_rainbow   gist_rainbow_r
gist_stern   gist_stern_r
gist_yarg   gist_yarg_r
gnuplot   gnuplot_r
gnuplot2   gnuplot2_r
gray   gray_r
hot   hot_r
hsv   hsv_r
inferno   inferno_r
jet   jet_r
magma   magma_r
nipy_spectral   nipy_spectral_r
ocean   ocean_r
pink   pink_r
plasma   plasma_r
prism   prism_r
rainbow   rainbow_r
seismic   seismic_r
spring   spring_r
summer   summer_r
tab10   tab10_r
tab20   tab20_r
tab20b   tab20b_r
tab20c   tab20c_r
terrain   terrain_r
twilight   twilight_r
twilight_shifted   twilight_shifted_r
viridis   viridis_r
winter   winter_r

Size

You can change the size of the dots with the s argument.

Just like colors, make sure the array for sizes has the same length as the arrays for the x- and y-axis:

Example

Set your own size for the markers:

import matplotlib.pyplot as plt
import numpy as np

x = np.array([5,7,8,7,2,17,2,9,4,11,12,9,6])
y = np.array([99,86,87,88,111,86,103,87,94,78,77,85,86])
sizes = np.array([20,50,100,200,500,1000,60,90,10,300,600,800,75])

plt.scatter(x, y, s=sizes)

plt.show()

Result:

Alpha

You can adjust the transparency of the dots with the alpha argument.

Just like colors, make sure the array for sizes has the same length as the arrays for the x- and y-axis:

Example

Set your own size for the markers:

import matplotlib.pyplot as plt
import numpy as np

x = np.array([5,7,8,7,2,17,2,9,4,11,12,9,6])
y = np.array([99,86,87,88,111,86,103,87,94,78,77,85,86])
sizes = np.array([20,50,100,200,500,1000,60,90,10,300,600,800,75])

plt.scatter(x, y, s=sizes, alpha=0.5)

plt.show()

Result:

Combine Color Size and Alpha

You can combine a colormap with different sizes of the dots. This is best visualized if the dots are transparent:

Example

Create random arrays with 100 values for x-points, y-points, colors and sizes:

import matplotlib.pyplot as plt
import numpy as np

x = np.random.randint(100, size=(100))
y = np.random.randint(100, size=(100))
colors = np.random.randint(100, size=(100))
sizes = 10 * np.random.randint(100, size=(100))

plt.scatter(x, y, c=colors, s=sizes, alpha=0.5, cmap='nipy_spectral')

plt.colorbar()

plt.show()

Result:

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