viz.scatter: for data frame function signature, inference about key orderings can be brittle

this works:

``` js
viz.scatter([{b: 1, a: 2},
             {b: 3, a: 5}])
```

but this doesn't:

``` js
viz.scatter([{a: 1, b: 2},
             {b: 3, a: 5}])
```
