Dissociate the xLabelWidth from the lines drawing

* For graphic with a lot of data, we can't display all xLabels. Now it's possible to display a fewLabels with a lot of value.
* Add method to specify the width xLabel.

@@ -57,7 +57,6 @@
 
 @property (nonatomic) BOOL showLabel;
 
-
 /**
  *  show CoordinateAxis ornot, Default is not
  */
@@ -73,4 +72,6 @@
  */
 @property (nonatomic, strong) NSString *yLabelFormat;
 
+- (void)setXLabels:(NSArray *)xLabels withWidth:(CGFloat)width;
+
 @end