Click on a cell in the resource histogram to open the utilization and availability pop up which provides detailed information about the histogram values.

Allocations graph

The allocations graph shows the level of allocation vs. availability vs. over allocation. 

The green fill shows the level of allocations and the blue fill the remaining capacity (before over allocation will occur). A red fill shows the level of over allocation.

Project allocation list

The projects that make up the allocations are displayed. Any causing an over allocation are indicated.