Obtaining values used in boxplot, using python and matplotlib

Question

I can draw a boxplot from data:

import numpy as np
import matplotlib.pyplot as plt

data = np.random.rand(100)
plt.boxplot(data)

Then, the

Answer 1

I've had this recently and have written a function to extract the boxplot values from the boxplot as a pandas dataframe.

The function is:

def get_box_plot_data(labels, bp):
    rows_list = []

    for i in range(len(labels)):
        dict1 = {}
        dict1['label'] = labels[i]
        dict1['lower_whisker'] = bp['whiskers'][i*2].get_ydata()[1]
        dict1['lower_quartile'] = bp['boxes'][i].get_ydata()[1]
        dict1['median'] = bp['medians'][i].get_ydata()[1]
        dict1['upper_quartile'] = bp['boxes'][i].get_ydata()[2]
        dict1['upper_whisker'] = bp['whiskers'][(i*2)+1].get_ydata()[1]
        rows_list.append(dict1)

    return pd.DataFrame(rows_list)

And is called by passing an array of labels (the ones that you would pass to the boxplot plotting function) and the data returned by the boxplot function itself.

For example:

import numpy as np
import matplotlib.pyplot as plt
import pandas as pd

def get_box_plot_data(labels, bp):
    rows_list = []

    for i in range(len(labels)):
        dict1 = {}
        dict1['label'] = labels[i]
        dict1['lower_whisker'] = bp['whiskers'][i*2].get_ydata()[1]
        dict1['lower_quartile'] = bp['boxes'][i].get_ydata()[1]
        dict1['median'] = bp['medians'][i].get_ydata()[1]
        dict1['upper_quartile'] = bp['boxes'][i].get_ydata()[2]
        dict1['upper_whisker'] = bp['whiskers'][(i*2)+1].get_ydata()[1]
        rows_list.append(dict1)

    return pd.DataFrame(rows_list)

data1 = np.random.normal(loc = 0, scale = 1, size = 1000)
data2 = np.random.normal(loc = 5, scale = 1, size = 1000)
data3 = np.random.normal(loc = 10, scale = 1, size = 1000)

labels = ['data1', 'data2', 'data3']
bp = plt.boxplot([data1, data2, data3], labels=labels)
print(get_box_plot_data(labels, bp))
plt.show()

Outputs the following from get_box_plot_data:

   label  lower_whisker  lower_quartile    median  upper_quartile  upper_whisker
0  data1      -2.491652       -0.587869  0.047543        0.696750       2.559301
1  data2       2.351567        4.310068  4.984103        5.665910       7.489808
2  data3       7.227794        9.278931  9.947674       10.661581      12.733275

And produces the following plot:

Answer 2

Why do you want to do so? what you are doing is already pretty direct.

Yeah, if you want to fetch them for the plot, when the plot is already made, simply use the get_ydata() method.

B = plt.boxplot(data)
[item.get_ydata() for item in B['whiskers']]

It returns an array of the shape (2,) for each whiskers, the second element is the value we want:

[item.get_ydata()[1] for item in B['whiskers']]