Transform data only if it changed

This is pretty much only a suggestion, I'm not sure about the consequences this would have.

The idea is to store the transformed data, and reuse it for later calls. For instance, when calling `draw` several times in a row without changing the data from some `resize` event. In this case is there a real benefit of retransforming the data again and again?

In the case of a resize, indeed, we call `draw` only to update the chart dimensions, not to process new data.

What do you think?