Set context of callback to data, up one level

Instead of setting the context to the stack function. Instead set it up one level in the data hierarchy.

So, for example, you could have data like:

```
var layers = [
  {
    "name": "apples",
    "values": [
      { "x": 0, "y":  91},
      { "x": 1, "y": 290}
    ]
  },
  {  
    "name": "oranges",
    "values": [
      { "x": 0, "y":  9},
      { "x": 1, "y": 49}
    ]
  }
];
```
And you set .values `function(d) {return d.values;}`

Then `.y` could reference the `names` with `this.names`