PointLabelCallback (Charba - J2CL and GWT charts library based on CHART.JS)

```
public interface PointLabelCallback
```
Callback function to transform data labels to point labels.

Author:

Andrea "Stock" Stocchero

Method Summary

All Methods Instance Methods Abstract Methods
Modifier and Type	Method	Description
`String`	`onCallback(Axis axis, String item, int index)`	Callback function to transform data labels to point labels.

- Method Detail
  - onCallback
```
String onCallback(Axis axis,
                  String item,
                  int index)
```
    Callback function to transform data labels to point labels. The default implementation simply returns the current string.
    
    Parameters:
    
    axis - axis instance where this callback as been defined
    
    item - label of current label
    
    index - index of the label
    
    Returns:
    
    new label to apply to point label