This appears to be a regression introduced in 1.11.0 - probably by: #53 and is still evident in "latest;

In 1.10.2 and earlier, when a legend was too wide and would overlap a pie chart, the legend would not show.
Since 1.11.0, the legend is shown overlapping the pie chart.

This can be replicated very easily with the following data declaration (default everything else):

var data = [{
    labels:['hello','this is a very long string and bad things are likely to happen', 'short string','moderately long string'],
    values:[4,4,4,4],
    type:'pie'
}];

Of these two behaviours, not showing the legend is preferred. The ideal solution (feature request I guess) would be to scale the legend to the a suitable non-overlapping size, then wrap the text within that.