For Chart.js 2.0+ you can use an array as label:
Quoting the DOCs:
“Usage: If a label is an array as opposed to a string i.e. [[“June”,”2015″], “July”] then each element is treated as a seperate line.”
var data = {
labels: [["My", "long", "long", "long", "label"], "another label",...],
...
}