Enable accessing input json data from callbacks

The issue is that my json contains plenty of metadata I need to access in various callbacks (tooltip, point size, color, etc.).

ATM, there is no way to do that. It would be awesome if targets contained some ID which could be used to access `config.data_json`, e.g.:

``` javascript
function get_tooltip(d, defaultTitleFormat, defaultValueFormat, color) {
var json_item = this.config.data_json[d.input_index];
...
```