Taskboard group by customization

Is it possible to alter the "Developer's Taskboard" view so that the items are grouped by a custom work item attribute. In this case, I have created an enumeration attribute called "function" (Development, Verification, Requirements, etc.)

If it's not possible to group by a custom attribute, then can I colorize the items by the value of this attribute? I see the their is an "Expression" which looks like the basis of which the taskboard uses to evaluate what should be what color i.e "type:Story"

How can I determine the "Expression" for my custom attribute? Is there any documentation that explain expressions?