This could be re-written as
```javascript
function yMin(series) {
  var seriesData = series.map(function(s) { return s.data.map(function(d) { return d.y }); });
  var yValues = d3.merge(seriesData);
  var yMin = Math.min.apply(Math, yValues);
  return yMin > 0 ? 0 : yMin;
}
```
Depends on if you want to save a line of code or not.