You should be able to use the canned report "Total Threat Events in EPO Database". I'm on 4.6 so it may not be available in other versions, if not here are the requirements:
Result Type: Threat Events
Chart Type: Pie chart
Labels Are: Event description - value (Descending)
Pie Slices Value: Number of Threat Events
Columns: Customize to your liking
Filter: No filter if you want all events or customize by events received time and date.