Dashboard - Invalid data in Graph

Dashboard - Invalid data in Graph

Hi,
In the Open Requests By Dashboard graph, I am getting the words Invalid data.  This occurs for any of the drop down fields.  The other Dashboard graphs are ok.  I have checked all Open Requests and made sure they have Priority, Mode & Category (Level is generally Null as we haven't started utilising this yet).  Where do I start looking for a solution?
 
Enterprise SD+, 5 technicians, W2K12

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