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When I create a barplot and overlay a bar chart using twin x then the boxes appear shifted by one to the right compared to the bars.

This problem has been identified before (Python pandas plotting shift x-axis if twinx two y-axes), but the solution no longer seems to work. (I am using Matplotlib 3.1.0) 

li_str = ['one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine', 'ten']

df = pd.DataFrame([[i]+j[k] for i,j in {li_str[i]:np.random.randn(j,2).tolist() for i,j in \
    enumerate(np.random.randint(5, 15, len(li_str)))}.items() for k in range(len(j))]
    , columns=['A', 'B', 'C'])

fig, ax = plt.subplots(figsize=(16,6))
ax2 = ax.twinx()
df_gb = df.groupby('A').count()
p1 = df.boxplot(ax=ax, column='B', by='A', sym='')
p2 = df_gb['B'].plot(ax=ax2, kind='bar', figsize=(16,6)
    , colormap='Set2', alpha=0.3, secondary_y=True)
plt.ylim([0, 20])

The problematic chart

The output shows the boxes shifted to the right by one compared to the bars. The respondent of the previous post rightly pointed out that the tick-locations of the bars are zero-based and the tick-locations of the boxes are one-based, which is causing the shift. However, the plt.bar() method the respondent uses to fix it, now throws an error, since an x-parameter has been made mandatory. If the x-parameter is provided it still throws an error because there is no parameter 'left' anymore. 

df.boxplot(column='B', by='A')
plt.twinx()
plt.bar(left=plt.xticks()[0], height=df.groupby('A').count()['B'],
  align='center', alpha=0.3)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-186-e257461650c1> in <module>
     26 plt.twinx()
     27 plt.bar(left=plt.xticks()[0], height=df.groupby('A').count()['B'],
---> 28         align='center', alpha=0.3)

TypeError: bar() missing 1 required positional argument: 'x'

In addition, I would much prefer a fix using the object-oriented approach with reference to the axes, because I want to place the chart into an interactive ipywidget.

Here is the ideal chart:

Ideal chart

Many thanks.

Tomerikoo
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nostalghia
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1 Answers1

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You can use the following trick: Provide the x-values for placing your bars starting at x=1. To do so, use range(1, len(df_gb['B'])+1) as the x-values.

fig, ax = plt.subplots(figsize=(8, 4))
ax2 = ax.twinx()
df_gb = df.groupby('A').count()
df.boxplot(column='B', by='A', ax=ax)
ax2.bar(range(1, len(df_gb['B'])+1), height=df_gb['B'],align='center', alpha=0.3)

enter image description here

Sheldore
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  • Ingenious simple trick. Thank you. But would you know how to make this change using object orientation. I tried – nostalghia Aug 12 '19 at 19:09
  • What exactly do you mean by "make this change using object orientation"? You mean getting the data from the axis object to place the bars? – Sheldore Aug 12 '19 at 19:10
  • Sorry, I am a new entrant in the forum and may not express myself well. I thought of assigning the twinned axis to an object via ax2=ax.twinx() and then have flexibility to assign the bars to ax2 when an event occurs via the pandas wrapper: df.bar(ax=ax2). I tried ax2.set_xticks(ax2.get_xticks()+1), but that seems to just shift the labels, not the bars. – nostalghia Aug 12 '19 at 19:31
  • @nostalghia : Check my edit. I basically replaced the `plt` in the last line by `ax2`. Is that what you are referring to? – Sheldore Aug 12 '19 at 19:36
  • Thank you for your efforts. It looks more like what I was after. It’s just getting too late over here. I need some sleep to get a clear head. I will check and confirm tomorrow morning. Cheers. – nostalghia Aug 12 '19 at 19:49
  • Many thanks, @Sheldore. Easier than I thought. It works fine, and I was successful in incorporating it into my larger code. – nostalghia Aug 13 '19 at 08:56