title: Stacked bar plot in R with multiple rows per day
body: <p>I would like to display work done in a day as a stacked bar plot, in order to see, day by day, how much activity I've done in each category, with the Y-axis representing time from 0:00 to 23:59.</p>
  
  <pre><code>#   day             tstart   tend   duration category
  1   2012-10-01      13:40    14:16  36       Recreation
  2   2012-10-02      10:15    10:57  42       Work
  3   2012-10-02      13:23    13:47  24       Chores
  4   2012-10-02      13:47    14:48  61       Work
  5   2012-10-03      09:09    11:40  151      Work
  6   2012-10-03      13:33    14:04  31       Recreation
  7   2012-10-03      17:00    19:40  160      Recreation
  </code></pre>
  
  <p>I know I will have to convert "time start" as a numeric, but I don't know how to "merge" the multiple rows for the same day, so that they're making up only one bar in the plot.</p>
  
  <p>In (very primitive) ASCII art, what I'm expecting is something like:</p>
  
  <pre><code>23:00
  22:00
  21:00
  20:00
  19:00                C
  18:00                C
  17:00                C
  16:00
  15:00
  14:00          W     R
  13:00    R     C
  12:00
  11:00                W
  10:00          W     W
   9:00                W
   8:00
   7:00
   6:00
   5:00
   4:00
   3:00
   2:00
   1:00
   0:00
          01    02    03
  </code></pre>
  
  <p>(where R, W and C would be bars of different colors for the different activites: Recreation, Work and Chores)</p>
  
  <p>In fact, being newbie in R plots, I don't know the plot function (and the plot package) I have to look at, moreover as they're will be holes in the plot -- no activity recorded (for example) between 0:00 and 09:09, then between 11:40 and 13:33, etc. on 2012-10-03...</p>