title: Pandas dataframe: Check if data is monotonically decreasing
body: <p>I have a pandas dataframe like this:</p>
  
  <pre><code>    Balance       Jan       Feb       Mar       Apr
  0  9.724135  0.389376  0.464451  0.229964  0.691504
  1  1.114782  0.838406  0.679096  0.185135  0.143883
  2  7.613946  0.960876  0.220274  0.788265  0.606402
  3  0.144517  0.800086  0.287874  0.223539  0.206002
  4  1.332838  0.430812  0.939402  0.045262  0.388466
  </code></pre>
  
  <p>I would like to group the rows by figuring out if the values from Jan through to Apr are monotonically decreasing (as in rows indexed 1 and 3) or not, and then add up the balances for each group, i.e. in the end I would like to end up with two numbers (1.259299 for the decreasing time series, and 18.670919 for the others).</p>
  
  <p>I think if I could add a column "is decreasing" containg booleans I could do the sums using pandas' groupby, but how would I create this column?</p>
  
  <p>Thanks,
  Anne</p>