bug - Pandas subplots loses data when sharex=True
Windows 7 64 OS, Python 3.7, fresh installation
```python
% matplotlib inline
import pandas as pd
import numpy as np
import matplotlib.pyplot as plot

ds1 = pd.DataFrame({ "Q2" : [1,2,1,0,2,1,6,4,2,1,2,1,3,2,1,5,1,2,1], 
    "Q3" : [1,5,1,2,2,5,7,1,1,1,5,1,4,2,5,4,1,1,1]})
ds2 = pd.DataFrame({ "Q2" : [1,5,1,4,2,1,5,1,2,1,2,1,3,2,3,4,1,2,1],  
    "Q3" : [3,5,1,2,2,1,4,1,3,1,3,1,4,4,1,3,1,3,1]})
ds3 = pd.DataFrame({ "Q2" : [1,1,2,5,3,3,4,4,2,1,2,1,3,2,3,5,3,4,1],  
    "Q3" : [3,2,1,0,2,1,5,1,2,1,2,1,5,2,1,0,1,2,1]})

ds = [ds1, ds2, ds3]

def func():    
    fig, axes = plot.subplots(len(ds), 5, figsize=(20, 20), sharex=True, sharey=True)
    for k, d in enumerate(ds):
        filter =  [ (d["Q2"] == 1),
                (d["Q2"] == 2),
                (d["Q2"] == 3),
                (d["Q2"] == 4),
                (d["Q2"] == 5)]

        for i, f in enumerate(filter):
            a = d[filter[i]].groupby(list(ds1)[1]).size()/len(d[filter[i]])
            a.plot(kind='bar',ax=axes[k][i])

```
Using a data array and filtering in certain manner, if plotting without sharex=True, the plots seems complete. Some data may be zero (or no data available), where no bar appears in this subplot (missing on the x-axis! - may be if sharex=False, but would be nicer if shown correctly; or could be switched on). - When I set sharex=True (nothing else is changed in my code!!), I do expect all information in all subplots, even if the value of certain data is zero. But instead of this, I do get just a single bar instead of my expected plots. My assumption is, that only the common bars (non zero) in all the plots are shown.

Is there any way to have all x-parameters shown in all subplots using the same x-axis ??
