title: Pandas - How to offset all values with if it less than previous value on whole dataframe
body: <p>I've got dataframe as follows:</p>
  
  <pre><code>time | value
  0       30
  1       40
  5       55
  10      10
  11      25
  20      10
  </code></pre>
  
  <p>As value stored should only increment (but sometimes it's being resetted) I want to create an output like follows;</p>
  
  <pre><code>0       30
  1       40
  5       55
  10      65   //offset 55
  11      80   //offset 55
  20      90   //offset 80
  </code></pre>
  
  <p>Any easy way to achieve it with pandas?</p>