Add valueFormatter to the Builder

Author: timeu

Diff for: dygraphs-gwt/src/main/java/com/github/timeu/dygraphsgwt/client/options/AxisOptions.java

@@ -135,6 +135,11 @@ public Builder pixelsPerLabel(int pixelsPerLabel) {
             return this;
         }
 
+        public Builder valueFormatter(ValueFormatter valueFormatter) {
+            this.options.valueFormatter = valueFormatter;
+            return this;
+        }
+
         public AxisOptions build() {
             return options;
         }

Diff for: dygraphs-gwt/src/main/java/com/github/timeu/dygraphsgwt/client/options/ValueFormatter.java

@@ -23,5 +23,5 @@ public interface ValueFormatter {
      * @param row The row of the data from which this point comes. g.getValue(row, 0) will return the x-value for this point.
      * @param col The column of the data from which this point comes. g.getValue(row, col) will return the original y-value for this point. This can be used to get the full confidence interval for the point, or access un-rolled values for the point.
      */
-    void onValueFormatter(long value,OptFunction opts,String seriesName,DygraphsJs dygraphjs,int row,int col);
+    String onValueFormatter(long value,OptFunction opts,String seriesName,DygraphsJs dygraphjs,int row,int col);
 }