Steps to reproduce :

1- Create a new user directory with one user.

2- Create a project with a few issues assign some of them to existing users and others unassigned
3- Assign an issue to the user part of the new user directory.
4- Run Pie chart on the project and choose "Assignee" for the filed "Statistics Type"
5- The user will be displayed with the counter set to 1
6- Disable the new Directory
7- The Pie chart will not display that user and the counter of the "unassigned will be increased by 1