How to vary the thickness of doughnut chart, using ChartJs.?

since version 2 the field has been renamed to cutoutPercentage.

cutoutPercentage
Number 50 – for doughnut, 0 – for pie
The percentage of the chart that is cut out of the middle.

It can be used like this

var options = {        
    cutoutPercentage: 40
};

more details here
http://www.chartjs.org/docs/#doughnut-pie-chart-chart-options

Update: Since version 3

var options = {        
    cutout: 40
};

According to the documentation at release notes of 3.x

Doughnut cutoutPercentage was renamed to cutout and accepts pixels as numer and percent as string ending with %.