Any question answered...

If I understand you correctly:

• The system cannot yet produce the data to drive the dashboard
  
• The dashboard, now, need only exist for the purposes of a 'demo', which I assume is some kind of proof of concept test / requirements validation / sales marketing thing
  
  

So instead of building something 'real' that is both open to interface and requirements changes, discover exactly what the dashboard need do for the demo then fake that using anything good enough. You'll need to discover what good enough looks like though, but some things I've used in the past, in order of increasing realism / complexity

• Paper sketches
  
• Powerpoint
  
• Photoshop
  
• Keynote
  
• Visio
  
• Axure
  
• Flash
  
• PHP CodeIgnitor & JQuery
  
• Raw Ruby + SQL & HTML / CSS
  
  

My understanding could be completely wrong though, YMMV etc.