Gadget PieChart could not render "Test Type"

Background
One of our customer just reported that there is a bug using XRay in combination with the Pie Chart Gadget which JIRA includes. Using the Pie Chart and try to display Test Types results in code for each value that should be rendered.

• JIRA: 6.4.1, running on Java 1.8
• XRay: 1.9.1

Steps to reproduce

1. Create Filter that shows "XRay Issues"
   • JQL I used:

     issuetype in (Test)

   • See Results
     
2. Create (new) dashboard
3. Add add gadget: Pie Chart by Atlassian
   • Select newly created filter
   • Select for Statistic Type: "Test Type"
   • Save gadget and see result:
     

Definition of Done / Expected Behavior
Test Types were rendered correctly.