Matplotlib: Plot on double y-axis plot misaligned

Question

I'm trying to plot two datasets into one plot with matplotlib. One of the two plots is misaligned by 1 on the x-axis. This MWE pretty much sums up the problem. What do I have to adjust to bring the box-plot further to the left?

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

titles = ["nlnd", "nlmd", "nlhd", "mlnd", "mlmd", "mlhd", "hlnd", "hlmd", "hlhd"]
plotData = pd.DataFrame(np.random.rand(25, 9), columns=titles)
failureRates = pd.DataFrame(np.random.rand(9, 1), index=titles)
color = {'boxes': 'DarkGreen', 'whiskers': 'DarkOrange', 'medians': 'DarkBlue', 
         'caps': 'Gray'}
fig = plt.figure()
ax1 = fig.add_subplot(111)
ax2 = ax1.twinx()
plotData.plot.box(ax=ax1, color=color, sym='+')
failureRates.plot(ax=ax2, color='b', legend=False)
ax1.set_ylabel('Seconds')
ax2.set_ylabel('Failure Rate in %')
plt.xlim(-0.7, 8.7)
ax1.set_xticks(range(len(titles)))
ax1.set_xticklabels(titles)
fig.tight_layout()
fig.show()

Actual result. Note that its only 8 box-plots instead of 9 and that they're starting at index 1.

William Miller · Accepted Answer · 2019-04-07 00:01:38Z

1

The issue is a mismatch between how box() and plot() work - box() starts at x-position 1 and plot() depends on the index of the dataframe (which defaults to starting at 0). There are only 8 plots because the 9th is being cut off since you specify plt.xlim(-0.7, 8.7). There are several easy ways to fix this, as @Sheldore's answer indicates, you can explicitly set the positions for the boxplot. Another way you can do this is to change the indexing of the failureRates dataframe to start at 1 in construction of the dataframe, i.e.

failureRates = pd.DataFrame(np.random.rand(9, 1), index=range(1, len(titles)+1))

note that you need not specify the xticks or the xlim for the question MCVE, but you may need to for your complete code.

answered Apr 7, 2019 at 0:01

William Miller

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Sheldore · Accepted Answer · 2019-04-06 21:35:15Z

1

You can specify the positions on the x-axis where you want to have the box plots. Since you have 9 boxes, use the following which generates the figure below

plotData.plot.box(ax=ax1, color=color, sym='+', positions=range(9))

answered Apr 6, 2019 at 21:35

Sheldore

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