Modification for BBT project

1. code learning tips
2. simplify getNearestEntryForXIndex
3. showNPoints
4. xLables related

Ported from bbt_bo_v1.7.3
Major Change:
	showNPoints is split into two parts
	One part in LineChart.java and the other in Transformer.java
	remove useless logs
Conflicts:
	MPChartLib/src/com/github/mikephil/charting/charts/BarLineChartBase.java
	MPChartLib/src/com/github/mikephil/charting/charts/LineChart.java