please try this:
1) create a new Trend, on Segmentation tab insert this formula: substring(/virtualMachine/internal//dateCreated,1,7)
[see help for XPATH on the internet]
2) on Dashboard create a new Trend widget (it's recommended that Mode is set to Multi-Trend Table):
Please let us know if it helps or if you need further info.
When I submitted this question I wasn't even aware it was using XPath. I have since had time to research and discover a few things, but when I initially tried the substring you have it would not return the dash. Now it is returning the full 7 characters. I don't understand that, but appreciate your respone.
My solution was the string below, but I tested your string again just to be sure and it worked this time.