How to get data when using datasets and transformations?

### Version
5.0.2

### Steps to reproduce
i cannot find a API to get the result after dataset has transformed.

### What is expected?
A API or another solution to get the transformed data.

### What is actually happening?
Before. we can use the aXis and series data length to control the datazoom.
Now. we use the dataset and transform to render the chart. we cannot get the data of current view. so we cannot to control the data-zoom through data.
Could you please give some suggestion to us to get the current view data (transformed).