Create Boxplots by Group in Matplotlib
matplotlib.pyplot.boxplot() & matplotlib.pyplot.setp() are the two useful functions to create grouped boxplots
Syntax: matplotlib.pyplot.boxplot(x, notch, positions, widths)
Syntax: matplotlib.pyplot.setp(obj, *args, **kwargs)
Python3
# import the matplotlib packageimport matplotlib.pyplot as plt# import the numpy packageimport numpy as np# create 2 - sample a 3-Dim array, that measures# the summer and winter rain fall amountsummer_rain = [[3, 5, 7], [15, 17, 12, 12, 15], [26, 21, 15]]winter_rain = [[16, 14, 12], [31, 20, 25, 23, 28], [29, 31, 35, 41]]# the list named ticks, summarizes or groups# the summer and winter rainfall as low, mid# and highticks = ['Low', 'Mid', 'High']# create a boxplot for two arrays separately,# the position specifies the location of the# particular box in the graph,# this can be changed as per your wish. Use width# to specify the width of the plotsummer_rain_plot = plt.boxplot(summer_rain, positions=np.array( np.arange(len(summer_rain)))*2.0-0.35, widths=0.6)winter_rain_plot = plt.boxplot(winter_rain, positions=np.array( np.arange(len(winter_rain)))*2.0+0.35, widths=0.6)# each plot returns a dictionary, use plt.setp()# function to assign the color code# for all properties of the box plot of particular group# use the below function to set color for particular group,# by iterating over all properties of the box plotdef define_box_properties(plot_name, color_code, label): for k, v in plot_name.items(): plt.setp(plot_name.get(k), color=color_code) # use plot function to draw a small line to name the legend. plt.plot([], c=color_code, label=label) plt.legend()# setting colors for each groupsdefine_box_properties(summer_rain_plot, '#D7191C', 'Summer')define_box_properties(winter_rain_plot, '#2C7BB6', 'Winter')# set the x label valuesplt.xticks(np.arange(0, len(ticks) * 2, 2), ticks)# set the limit for x axisplt.xlim(-2, len(ticks)*2)# set the limit for y axisplt.ylim(0, 50)# set the titleplt.title('Grouped boxplot using matplotlib') |
Output:
Explanation:
- Import the necessary packages numpy and matplotlib.
- Create 2 – sample arrays of 3 dimensions named, summer_rain and winter_rain
- Create another list named ticks, that summarizes or groups the summer and winter rainfall as low, mid, and high.
- Create a boxplot for two arrays separately as shown.
- Use the position argument to specify the location of every box in the group, here, the summer_rain plot has 3 boxes which are separated at a spacing of [-0.35, 1.65, 3.65] and the winter_rain plot has 3 boxes which are separated at a spacing of [0.35, 2.35, 4.35].
- The width of each box is kept at 0.6.
- Now, each individual plot summer_rain_plot and winter_rain_plot returns a dictionary, This dictionary has a list of properties of the box plot like whiskers, median, fliers etc.
- Now, iterate through the dictionary items and use plt.setp() function to assign a unique color code for each group as shown.
- Use plt.plot() function to draw a default line to represent the legends of the box plot.
- The define_box_properties function, takes the plot, color and the legend name as arguments and set the properties of the plot appropriately.
- Finally, to improve the aesthetic value, use xlim, ylim function to define the limits of the x and y axis and use xticks function to label the x-axis. Set the title of the plot using plt.title() function.
How to Create Boxplots by Group in Matplotlib?
Boxplots by groups can be created using the matplotlib package, but, however, if you wish to make more customizations to your grouped box plot, then the seaborn package provides a go-to function that supports a wide variety of customizations to the grouped box plots. Matplotlib doesn’t provide an explicit function to create a grouped box plot. We have to construct the plot as per the required format. This article discusses how to create grouped boxplots in matplotlib.
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