Suggest to add a clear data method

Now I have to define a function as:

```
function Clear(chart) {
chart.data().forEach(function (target) {
var length = target.values.length;
for (var i = 0; i < length; i++) {
target.values[i].value = null;
}
});
}
```

Start by locating the chart.data() entry point and the handling of target.values shown in the issue. Determine how a clear-data method should fit the chart API, then verify that it resets the existing data values without requiring the workaround shown. The issue names no implementation file or test, so those must be found in the repository.

Written by the indexing model from the issue text.