Basically is the same issue describe in here: https://ecosystem.atlassian.net/browse/JCHART-440
I have done further testing based in JIRA 6.0 and JIRA 6.0.1 wth JIRA Charting Plugin v1.12 and 1.16 both still having the same issue.
Create two project Test1 & Test2
Create 1 issue with original estimated time each
Both of the project is using Default Permission Scheme
Edit the Browse Projects permission to Group: Anyone so that anonymous user can see both of the project
Then edit the User 'Interface > System Dashboard' to include 2 workload pie chart gadget (1 pointing to Test1 project and another to Test2 project)
View the dashboard while still login to ensure both the pie chart is showing.
Logout and view the dashboard as anonymous and you will get the following:
and also it will produce the following stack traces in atlassian-jira.log file:
Side Note: This problem is not reproducible in JIRA 5.2.11