title: How to calculate slope of a dataframe, upto a specific row number?
body: <p>I have this data frame that looks like this:</p>
  <pre><code>PE  CE  time
  0   362.30  304.70  09:42
  1   365.30  303.60  09:43
  2   367.20  302.30  09:44
  3   360.30  309.80  09:45
  4   356.70  310.25  09:46
  5   355.30  311.70  09:47
  6   354.40  312.98  09:48
  7   350.80  316.70  09:49
  8   349.10  318.95  09:50
  9   350.05  317.45  09:51
  10  352.05  315.95  09:52
  11  350.25  316.65  09:53
  12  348.63  318.35  09:54
  13  349.05  315.95  09:55
  14  345.65  320.15  09:56
  15  346.85  319.95  09:57
  16  348.55  317.20  09:58
  17  349.55  316.26  09:59
  18  348.25  317.10  10:00
  19  347.30  318.50  10:01
  </code></pre>
  <p>In this data frame, I would like to calculate the slope of both the first and second columns separately to the time period starting from (say in this case is 09:42 which is not fixed and can vary) up to the time 12:00.
  please help me to write it..</p>