To display a graph as a pie chart in the dashboard you need to click on...

1. Analysis View
2. Click on the chart in the Analysis Tree
3. Click on the View tab
4. Click on the Set Graph Appearance toolbar icon
http://screencast.com/t/9FICpmF5Jk
5. Click on the Appearance Tab
6. Change the Default Layout to Pie Chart
7. Click OK
http://screencast.com/t/bqXpJXa08

You will need to refresh the dashboard for changes to take affect