Fix for donut chart when a series only has one non-zero value

If a data series had only one non-zero value that was not the first element, such as [0, 0, 23, 0, 0], it would incorrectly set the startAngle. This fix checks the series first then updates the startAngle calculation to take that into consideration.
Cool stuff :-) Thanks for working on this and using Array extras! In Chartist we follow the convention to write function bodies on a new line. Can you put the return statement on a new line something like this?

```javascript
var hasSingleValInSeries = data.series.filter(function(val) {
  return val !== 0; 
}).length === 1;
```