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You should use tooltipTemplate option:
var options = {
// String - Template string for single tooltips
tooltipTemplate: "<%= label %> January: <%= value %>"
};
window.myLine = new Chart(ctx).Line(data, options);
Here is sample: https://jsfiddle.net/wckfhrsa/7/
Also, check more tooltip options global chart configuration
Source:stackexchange.com