added visible data api

[mawsyh]

exposed the API to get data of the visible part of the chart

[liihuu]

getVisibleRange()?

[mawsyh]

Correct if I'm wrong but visibleRange returns a single object with from and to values, but i needed the array of visible data to do some mathematical calculations on them

[liihuu]

From and to are KLineData array index, only need to get the current data through getDataList.

[mawsyh]

Thanks but in some cases I have over 10000 objects in my data, fetching them all and slicing it doesn't seem the best approach to me.