Combining Date Fields on Charts

[johncat]

I have the following data rows being returned from a json webservice:

bugId, OpenDate, CloseDate

I would like to plot a line chart with date along the x-axis and a count of the number of bugs opened on that date and the number of bugs closed on that date, is this possible in jasper reports?  I can create two separate charts using the html5 chart:

 - level = OpenDate (or CloseDate)

 - measure = count(bugId)

What I am unable to do is combine the date fields, essentially I need UNION two queries to pull back open tickets and closed tickets as seperate rows but as this is a REST API I cannot achieve this in the datasource so are looking for alternative options within Jasper Reports itself