title: rrdtool does not keep max
body: <p>I create a RRD with one DS, the processing time of a script, with 10 minutes step.
  Having three archives, with MAX as aggregation function : all values over a week, every hour over a month, every day over two years.</p>
  
  <pre><code>rrdtool create RRD --start 1411561343 --step 600s \
  DS:processtime:GAUGE:1200:0:U \
  RRA:MAX:0.5:1:1008 \
  RRA:MAX:0.5:6:744 \
  RRA:MAX:0.5:144:732
  </code></pre>
  
  <p>I <a href="https://framadrop.org/r/GzgDyH1KAp#4HxzKgLPyh/Ej1hfXNo5oWZyjx1HoWGXFwi1a0DYud4=" rel="nofollow">populate</a> it from a file which contains all records from the 2014/09/24 at 14:32:23 (1411561943) to the 2016/01/11 at 11:07:25 (1452503245).
  The maximum is for the 2015/09/11 at 14:18:35 (1441973915), 23340.</p>
  
  <p>When I graph or <a href="https://framadrop.org/r/cljLItNzSu#b/TkDVEqEGvfj+JaJ+ITiY4NrJkdmVKg9q4eydkfNVI=" rel="nofollow">dump</a> the rrd, I have a lot of NaN, I don't see this maximum, neither many other significant values.
  The max I have in the rrd is <code>&lt;!-- 2015-08-06 02:00:00 CEST / 1438819200 --&gt; &lt;row&gt;&lt;v&gt;8.0004250000e+02&lt;/v&gt;&lt;/row&gt;</code>.</p>
  
  <p>Is it related to the fact that intervals are not exactly 10 minutes but between 8 and 12 ?</p>
  
  <p>If so, it there a way to change this behavior ?</p>