Default min/max y-axis values for BandCharts

I'm using the BandChart (pulled from the current master branch), and when panning or zooming the chart I get a lot of these:

```
Error: attribute d: Expected number, "…491151407752467,NaNL0.0296717327…".
```

For some reason it seems that the [areaGenerator](https://github.com/esnet/react-timeseries-charts/blob/master/src/components/BandChart.js#L429) frequently produces NaNs for my data, all of which is between 0 and 1. I hacked together a solution for myself by providing default values for the yaxis of 0 and 1:

const areaGenerator = area()
.curve(curves[this.props.interpolation])
.x(d => d.x0)
.y0(d => d.y0 || 0)
.y1(d => d.y1 || 1);

Should there be a way to specify min/max values for the y axis here? Perhaps there's a better approach to this. If so, I'd be happy to help contribute.

In case it helps I'm constructing my data with something like this:

const series = new TimeSeries({
name: "Data",
columns: ['index', 'value', 'bands'],
points: [
["1s-1438527040", 0.3, [0.2, 0.3, 0.4]],
...
]
})