Q:

Matplotlib, plotting pandas series: AttributeError: 'tuple' object has no attribute 'xaxis'

I am plotting Pandas Series data, which records the sum of "events" each week in 1981. The series is named 'weekly_data'.
1981-03-16    1826
1981-03-23    1895
1981-03-30    1964
1981-04-06    1978
1981-04-13    2034
1981-04-20    2073
1981-04-27    2057
dtype: int64

I would like to place ticks by year, and by week. When I try to plot this, I receive an AttributeError:
fig = plt.figure(figsize=(12,5))
ax = plt.subplots(111)
plt.plot(weekly_data, color = 'green' )
yloc = YearLocator()
mloc = MonthLocator()
ax.xaxis.set_major_locator(yloc)
ax.xaxis.set_minor_locator(mloc)
ax.grid(True)
plt.show()

The error is
AttributeError                            Traceback (most recent call last)
<ipython-input-92-843dbab30ed7> in <module>()
      6 yloc = YearLocator()
      7 mloc = MonthLocator()
----> 8 ax.xaxis.set_major_locator(yloc)
      9 ax.xaxis.set_minor_locator(mloc)
     10 ax.grid(True)

AttributeError: 'tuple' object has no attribute 'xaxis'

How can I fix this? 
EDIT: Following Mike Mueller, the error above was plt.subplot(111). However, I still cannot get weekly ticks to work. Perhaps we need to use ax.set_xticks(major_ticks) or  ax.set_xticks(minor_ticks, minor=True) 
Here is the pandas series data I am plotting from 1991
Datetime
1990-12-23    1980
1990-12-30    1860
1991-01-06    1761
1991-01-13    1792
1991-01-20    1825
....
dtype: int64

and this is the code
fig = plt.figure(figsize=(12,5))
ax = plt.subplot(111)
plt.plot(weekly_data1991, color = 'green' )
yloc = YearLocator()
mloc = MonthLocator()
ax.xaxis.set_major_locator(yloc)
ax.xaxis.set_minor_locator(mloc)
ax.grid(True)
plt.show()

Here is the plot output

I am confused myself

A:

You are creating 111 subplots. Change:
ax = plt.subplots(111)

into:
ax = plt.subplot(111)

and you it should work. It creates one subplot array of one row, one column, and one subplot. For example, this:
plt.subplot(231)
plt.subplot(236)

creates subplot 1 and subplot 6 in array of 2 rows and 3 columns:
 
You make the months visible with:
ax.xaxis.set_minor_formatter(matplotlib.dates.DateFormatter('%B'))

Th year an the month are printed on top of each other. One solution is to have the years on the top and the months on bottom:
ax.xaxis.set_tick_params(labeltop='on', labelbottom='off')

Use:
mloc = matplotlib.dates.MonthLocator(range(1, 12, 4))

to show only every fourth month.

