Can't get labelFormatter working on pie chart?

[andig]

I'm using the current CVS version and am trying something like this:
var f = Flotr.draw($('chart_genres'), [ ..data..
], { ...
pie: {show: true},
labelFormatter:
function (slice) {
return (slice.name);
}, ...

For some reason however the labelFormatter is never executed, chart
still shows percentages.

Any idea what I'm doing wrong?

Best,
Andi