Pie chart update data set

* [x] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## What did you do?

I tried to update the initial values of the pie chart using below code
` graphView.data = newData`
`graphView.animate(yAxisDuration: 1)`
`graphView.notifyDataSetChanged()`

## What did you expect to happen?

Expected to happen a smooth animation on the updated chart slices.

## What happened instead?

Entire pie chart redrawn instead of updating only the slices I have updated. I just want to update only the changed slices not the entire chart. Please help to fix this.

Start by reproducing the pie-chart update with graphView.data = newData, followed by animate(yAxisDuration: 1) and notifyDataSetChanged(). Trace how those calls redraw the chart and compare the requested behavior with the current update path. Done means updating only changed slices with a smooth animation rather than redrawing the entire pie chart.

Written by the indexing model from the issue text.