[FIXED] Boxplot by two groups in pandas

Issue

I have the following dataset:

df_plots = pd.DataFrame({'Group':['A','A','A','A','A','A','B','B','B','B','B','B'],
                         'Type':['X','X','X','Y','Y','Y','X','X','X','Y','Y','Y'],
                         'Value':[1,1.2,1.4,1.3,1.8,1.5,15,19,18,17,12,13]})
df_plots
    Group   Type    Value
0   A       X       1.0
1   A       X       1.2
2   A       X       1.4
3   A       Y       1.3
4   A       Y       1.8
5   A       Y       1.5
6   B       X       15.0
7   B       X       19.0
8   B       X       18.0
9   B       Y       17.0
10  B       Y       12.0
11  B       Y       13.0

And I want to create boxplots per Group (there are two in the example) and in each plot to show by type. I have tried this:

fig, axs = plt.subplots(1,2,figsize=(8,6), sharey=False)
axs = axs.flatten()

for i, g in enumerate(df_plots[['Group','Type','Value']].groupby(['Group','Type'])):
    g[1].boxplot(ax=axs[i])

• Results in an IndexError, because the loop tries to create 4 plots.

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-12-8e1150950024> in <module>
      3 
      4 for i, g in enumerate(df[['Group','Type','Value']].groupby(['Group','Type'])):
----> 5     g[1].boxplot(ax=axs[i])

IndexError: index 2 is out of bounds for axis 0 with size 2

Then I tried this:

fig, axs = plt.subplots(1,2,figsize=(8,6), sharey=False)
axs = axs.flatten()

for i, g in enumerate(df_plots[['Group','Type','Value']].groupby(['Group','Type'])):
    g[1].boxplot(ax=axs[i], by=['Group','Type'])

But no, I have the same problem. The expected result should have only two plots, and each plot have a box-and-whisker per Type. This is a sketch of this idea:

Please, any help will be greatly appreciated, with this code I can control some aspects of the data that I can't with seaborn.

Solution

As @Prune mentioned, the immediate issue is that your groupby() returns four groups (AX, AY, BX, BY), so first fix the indexing and then clean up a couple more issues:

1. Change axs[i] to axs[i//2] to put groups 0 and 1 on axs[0] and groups 2 and 3 on axs[1].
2. Add positions=[i] to place the boxplots side by side rather than stacked.
3. Set the title and xticklabels after plotting (I'm not aware of how to do this in the main loop).

for i, g in enumerate(df_plots.groupby(['Group', 'Type'])):
    g[1].boxplot(ax=axs[i//2], positions=[i])

for i, ax in enumerate(axs):
    ax.set_title('Group: ' + df_plots['Group'].unique()[i])
    ax.set_xticklabels(['Type: X', 'Type: Y'])

---

Note that mileage may vary depending on version:

	matplotlib.__version__	pd.__version__
confirmed working	3.4.2	1.3.1
confirmed not working	3.0.1	1.2.4

Answered By - tdy