title: January minor tick label is not showing in matplotlib
body: <p>I am plotting data for a time series, and have major tick marks for the year and minor for the month. Everything looks fine, except for '01' for January is not appearing over the year. Does anyone have any advice on how to fix this? Thank you!</p>
  
  <p>Code and the relevant graph output is below.</p>
  
  <pre><code>
  fig=plt.figure(figsize=(40,30))
  ax1 = plt.subplot(411)
  plt.title('Moving Window Plot: Downsview 2015-2020',size=30)
  ax2 = plt.subplot(412)
  ax3 = plt.subplot(413)
  
  
  ax1.errorbar(ym_grp.index,ym_grp.slope, yerr=ym_grp_sem.slope, ls='none',marker = 'o')
  ax2.errorbar(ym_grp.index,ym_grp.yint, yerr = ym_grp_sem.yint, ls='none',marker = 'o')
  ax3.errorbar(ym_grp.index,ym_grp.r2, yerr =ym_grp_sem.r2, ls='none', marker = 'o')
  
  ax1.xaxis.set_major_locator(matplotlib.dates.YearLocator())
  ax1.xaxis.set_minor_locator(matplotlib.dates.MonthLocator())
  
  ax1.xaxis.set_major_formatter(matplotlib.dates.DateFormatter("\n%Y"))
  ax1.xaxis.set_minor_formatter(matplotlib.dates.DateFormatter("%m"))
  plt.setp(ax1.get_xticklabels(), rotation=0, ha="center")
  
  ax3.set_xlabel('Time (UTC)',fontsize=20)
  ax1.set_xlim()
  
  ax1.set_ylabel('ΔCO/ΔCO₂',fontsize=20)
  ax2.set_ylabel('offset',fontsize=20)
  ax3.set_ylabel('r-squared',fontsize=20)
  </code></pre>
  
  <p>Output:</p>
  
  <p><a href="https://i.stack.imgur.com/KG2rR.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KG2rR.png" alt="graph output"></a></p>