Y2 behaves oddly

I get the feeling `y2` needs better documentation, because it doesn't behave at all like `y`. Demo: http://plnkr.co/edit/XFUu193qu4F8dIfvG4L3?p=preview

They're the exact same graph, except the `y`-axis is set to `data1` and `data2` in the first and second graphs, and then `y2` in the third and fourth (respectively).

Observations:
1. The values on the `y`-axis don't seem to change, regardless of which column is associated with said-axis.
2. Associating `y2` with a column seems to remove the first column's line, regardless of which column is associated with `y2`.
3. The `y2` axis values change to reflect the max/min of the column associated with it.
4. Though not shown in the Plunker, if you don't associate a column with `y`, it will use the entire dataset's min/max. If you show `y2` without associating a column, its values range from 1 to 0.

While I get the feeling `y2` isn't intended to be used by itself, it's currently pretty confusing to use.

I'm marking as both `bug` and `documentation needed` given I'm not sure which it is — @masayuki0812, feel free to untag one or the other, depending on how you see it.