[Chartjs]-Format X label on mouse hover in chart.js

Edit the title callback property in your tooltips attribute, stored in the option :

tooltips: {
    callbacks: {
        title: function(tooltipItem, chartData) {
            // You return what you want here
            return "Title";
        }
    }
}

You can see on this jsFiddle your updated example, and here is its result :