ChartYAxisValueFormatter in iOS

[chuynadamas]

Hi Daniel,
First of all i want to thank you, this very handful API.

In other hand i want to implement an YAxis large formatter just as you do in the example for the android version, I saw that you used the class YAxisValueFormatter which is missing in the iOS version. Anyway i tired to reimplement the Formatter that you have in the ChartDemo project and this uses the ChartXAxisValueFormatter protocol which i guess only working for the XAxis values.

Is there anyway that we can format the YAxis value just as you did in the Android Version, with a ChartXAxisValueFormatter ? or I need to implement a NSNumberFormatter ?

Thanks in advance.

[chuynadamas]

Please refer #561