Add label to yAxis

Credits to wangshijun2010, pull request shutterstock#404

src/js/Rickshaw.Graph.Axis.Y.js

@@ -3,7 +3,8 @@ Rickshaw.namespace('Rickshaw.Graph.Axis.Y');
 Rickshaw.Graph.Axis.Y = Rickshaw.Class.create( {
 
 	initialize: function(args) {
-
+
+    this.args = args;
 		this.graph = args.graph;
 		this.orientation = args.orientation || 'right';
 
@@ -101,6 +102,21 @@ Rickshaw.Graph.Axis.Y = Rickshaw.Class.create( {
 			.attr("transform", transform)
 			.call(axis.ticks(this.ticks).tickSubdivide(0).tickSize(this.tickSize));
 
+    // add label
+    if (this.args.label && this.args.label.text) {
+      var label = this.args.label;
+      this.vis.append("text")
+        .attr("class", "axis-label")
+        .attr("text-anchor", "end")
+        .attr("y", label.offsetX || "1em")
+        .attr("x", label.offsetY || "1em")
+        .style("color", label.color || "black")
+        .style("opacity", label.opacity || "0.5")
+        .style("font-size", label.fontSize || "10px")
+        .attr("transform", "rotate(-90)")
+        .text(label.text);
+    }
+
 		return axis;
 	},