Added custom Point value formatter

[GoogleCodeExporter]

I created a formatter interface with a single method to format the Point value 
in the chart.
You can set the Object implementing the interface in the SimpleSeriesRenderer. 
If the formatter is set, the XYChart will use this formatter otherwise will use 
classic method. 
I created my custom implementation too. You can set 2 text formats (2 Strings), 
one for each signed number (positive numbers and negative numbers).

Attached:
-IValueFormatter the interface in the package org.achartengine.formatter
-SignedNumberFormatter my implementation in the package 
org.achartengine.formatter
-The patch for class XYChart.java
-The patch for class SimpleSeriesRenderer.java

Original issue reported on code.google.com by matteo.l...@gmail.com on 9 Mar 2015 at 10:44

Attachments:

• IValueFormatter.java
• SignedNumberFormatter.java
• XYChart.java.patch