Ok, is not about the legend widget, the problems is with the data you used.
In order to display the percent values, you need to define the legend text like this:
var chartData = [
{x:1,y:5,legend:"Income"},
{x:1,y:10,legend:"Outcome"},
];
With this data, the percent values will be calculated and displayed on the chart, and the legend widget will display the proper labels