title: Pandas Compare Similar DataFrames and get Min
body: <p>Given the following data frames:</p>
  
  <pre><code>d1=pd.DataFrame({'A':[1,2,np.nan],'B':[np.nan,5,6]})
  d1.index=['A','B','E']
  
      A       B
  A   1.0     NaN
  B   2.0     5.0
  E   NaN     6.0
  
  d2=pd.DataFrame({'A':[4,2,np.nan,4],'B':[4,2,np.nan,4]})
  d2.index=['A','B','C','D']
      A       B
  A   4.0     4.0
  B   2.0     2.0
  C   NaN     NaN
  D   4.0     4.0
  </code></pre>
  
  <p>I'd like to compare them to find the lowest value in each corresponding row, while preserving all rows indices from both.
  Here is the result I'm looking for:</p>
  
  <pre><code>    A       B
  A   1.0     4.0
  B   2.0     2.0
  C   NaN     NaN
  D   4.0     4.0
  E   NaN     6.0
  </code></pre>
  
  <p>Thanks in advance!</p>