This is the answer, with discrete and hAxis,
new Chartkick.LineChart("chart-2",
{"2013-02-10": 11, "2013-02-11": 6, "2013-02-12": 12, "2013-02-13": 5},
{"library":
{title: "Company Performance",
"hAxis":{"title":"Date","gridlines":
{"count":3,"color":"#CCC"},"format":"dd/MM/yy"}},
"discrete": true});