The underlying charting engine seems to not support that configuration for that type of chart. However, you can add the following attribute instead on the tm1-ui-chart directive:
Which should give you this:
For more options with the pie chart for example, you can check it out here:
The site has plunker as well for which you can edit it live, and see the effects straight away on the Plunker linked to the charts.
Was this the value that you were looking for?