How to set ChartJS Y axis title?

In Chart.js version 2.0 this is possible:

options = {
  scales: {
    yAxes: [{
      scaleLabel: {
        display: true,
        labelString: 'probability'
      }
    }]
  }
}

See axes labelling documentation for more details.

For x and y axes:

     options : {
      scales: {
        yAxes: [{
          scaleLabel: {
            display: true,
            labelString: 'probability'
          }
        }],
        xAxes: [{
          scaleLabel: {
            display: true,
            labelString: 'hola'
          }
        }],
      }
    }

For Chart.js 2.x refer to andyhasit's answer - https://stackoverflow.com/a/36954319/360067

For Chart.js 1.x, you can tweak the options and extend the chart type to do this, like so

Chart.types.Line.extend({
    name: "LineAlt",
    draw: function () {
        Chart.types.Line.prototype.draw.apply(this, arguments);

        var ctx = this.chart.ctx;
        ctx.save();
        // text alignment and color
        ctx.textAlign = "center";
        ctx.textBaseline = "bottom";
        ctx.fillStyle = this.options.scaleFontColor;
        // position
        var x = this.scale.xScalePaddingLeft * 0.4;
        var y = this.chart.height / 2;
        // change origin
        ctx.translate(x, y);
        // rotate text
        ctx.rotate(-90 * Math.PI / 180);
        ctx.fillText(this.datasets[0].label, 0, 0);
        ctx.restore();
    }
});

calling it like this

var ctx = document.getElementById("myChart").getContext("2d");
var myLineChart = new Chart(ctx).LineAlt(data, {
    // make enough space on the right side of the graph
    scaleLabel: "          <%=value%>"
});

Notice the space preceding the label value, this gives us space to write the y axis label without messing around with too much of Chart.js internals

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Fiddle - http://jsfiddle.net/wyox23ga/

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