title: How to add a new column and aggregate values in R
body: <p>I am completely new to gnuplot and am only trying this because I need to learn it. I have a values in three columns where the first represents the filename (date and time, one hour interval) and the remaining two columns represent two different entities Prop1 and Prop2.</p>
  
  <pre><code>Datetime             Prop1        Prop2
  
  20110101_0000.txt     2            5
  20110101_0100.txt     2            5
  20110101_0200.txt     2            5
  ...
  20110101_2300.txt     2            5
  20110201_0000.txt     2            5
  20110101_0100.txt     2            5
  ...
  20110201_2300.txt     2            5
  ...
  </code></pre>
  
  <p>I need to aggregate the data by the hour of the day (the **_0100) which is the last four numeric digits. So, I want to create another column called hour which tells me the hour of the day. It means <code>0000 = 0h, 0100 = 1h, ...... 2200 = 22h</code> etc. </p>
  
  <p>I then want to get the sum of Prop1 and Prop2 for each hour, so in the end get something like.</p>
  
  <pre><code>Hour  Prop1   Prop2
  0h     120     104
  1h     230     160
  ...
  10h    90      110
  ...
  23h    100    200 
  </code></pre>
  
  <p>and the get a line plot of Prop1 and Prop2.</p>