This is the donut chart. It's essentially a pie chart with a big hole in the middle. You can use the pie chart properties when customising the donut.
The example file is here.
<script>
window.onload = function ()
{
var data = [45,57,48,32];
var donut = new RGraph.Donut('myDonut', data);
donut.Set('chart.labels', ['Jan', 'Ben', 'Mark', 'Lucy']);
donut.Set('chart.linewidth', 5);
donut.Set('chart.strokestyle', 'rgba(255,255,255,1)');
donut.Set('chart.tooltips', ['Jan', 'Ben', 'Mark', 'Lucy']);
donut.Draw();
}
</script>
Please see the Pie chart documentation for what properties you can set. Most of the pie chart properties are available on the donut chart too.