title: Sum two graphs, when the second RRD file is started only now
body: <p>I don't know how to explain my problem.... but
  I have two RRD files:</p>
  
  <pre><code>a.rrd
  b.rrd
  </code></pre>
  
  <p>I'm trying to sum both of the files and STACK them up in the graph.
  like:</p>
  
  <pre><code>my $bla = RRDs::graph "-",
  
      "--title","Test",
      "--imgformat=PNG",
      "--width=680",
      "--height=200",
  
      "DEF:Default0_=a.rrd:default:AVERAGE",
      "DEF:Real0_=a.rrd:real:AVERAGE",
  
      "DEF:Default1_=b.rrd:default:AVERAGE",
      "DEF:Real1_=b.rrd:real:AVERAGE",
  
  
      "CDEF:Default=Default0_,Default1_,+",        
      "CDEF:Real=Real0_,Real1_,+",
  
  
      'AREA:Default#00CF00:Default Test',
      'GPRINT:Default:MIN:Min\: %10.0lf%s',
      'GPRINT:Default:MAX:Max\: %10.0lf%s',
      'GPRINT:Default:AVERAGE:Average\: %10.0lf%s',
      'GPRINT:Default:LAST:Current\: %10.0lf%s \l',
  
      'STACK:Real#006699:Real Test',
      'LINE2:Real#000000',
      'GPRINT:Real:MIN:Min\: %10.0lf%s',
      'GPRINT:Real:MAX:Max\: %10.0lf%s',
      'GPRINT:Real:AVERAGE:Average\: %10.0lf%s',
      'GPRINT:Real:LAST:Current\: %10.0lf%s \l',
  </code></pre>
  
  <p>And my Result is:</p>
  
  <p><a href="http://www.freeimagehosting.net/uploads/8d99a4a675.jpg" rel="nofollow noreferrer">alt text http://www.freeimagehosting.net/uploads/8d99a4a675.jpg</a></p>
  
  <p>problem: it doesn't print the values from file a.rrd, it display the graph only from the position of the b.rrd file.</p>
  
  <p>instide of something like this ( only the first part will be with zeros ):</p>
  
  <p><a href="http://www.freeimagehosting.net/uploads/e036f93797.jpg" rel="nofollow noreferrer">alt text http://www.freeimagehosting.net/uploads/e036f93797.jpg</a></p>
  
  <p>Obviously, this is because the second graph doesn't have unix timestamp when the first graph does.</p>
  
  <p>so how can i fill it with zeros ? or change my graph conf ?</p>