This should do the trick:
d3.csv("wlythree.csv", function(data) {
var values = [];
values = data.map(function(d) { return d.percentage; });
console.log(values)
});
It yields:
["0.275862068965517", "0.137931034482759", ...]
EDIT: placed declaration of values
inside the callback since the csv loading is asynchronous and that better reflects the script flow.