Broken Linear regression

Hi,

The linear regression implementation return Nan for the following data:

```
var xyData = [
                { x: 0, y: 2.0},
                { x: 1, y: 2.0},
                { x: 2, y: 2.0},
            ];
```

Your implementation divides by the stddev, thus, not allowing it to be 0.

I think that linear regression should return a solution for any data points.
