Make delegate methods optional

They should be optional methods so we can get rid of warning when only using one kind of these chart.

@@ -9,7 +9,7 @@
 #import <Foundation/Foundation.h>
 
 @protocol PNChartDelegate <NSObject>
-
+@optional
 /**
  * Callback method that gets invoked when the user taps on the chart line.
  */
@@ -27,4 +27,4 @@
  */
 - (void)userClickedOnBarAtIndex:(NSInteger)barIndex;
 
-@end
\ No newline at end of file
+@end