参考： http://null-point.iteye.com/blog/1591896
用aChartEngine画TimeChart的时，横坐标的日期显示格式很多样。下面这些例子加上一些变通基本上涵盖了所有的格式：
“
- For the month of September:
- M -> 9
- MM -> 09
- MMM -> Sep
- MMMM -> September
- For 7 minutes past the hour:
- m -> 7
- mm -> 07
- mmm -> 007
- mmmm -> 0007
- Examples for April 6, 1970 at 3:23am:
- "MM/dd/yy h:mmaa" -> "04/06/70 3:23am"
- "MMM dd, yyyy h:mmaa" -> "Apr 6, 1970 3:23am"
- "MMMM dd, yyyy h:mmaa" -> "April 6, 1970 3:23am"
- "E, MMMM dd, yyyy h:mmaa" -> "Mon, April 6, 19703:23am&
- "EEEE, MMMM dd, yyyy h:mmaa" -> "Monday, April 6, 1970 3:23am"
- "'Noteworthy day: 'M/d/yy" -> "Noteworthy day: 4/6/70"
”