Add API for switching axes

src/api.axis.js

@@ -58,3 +58,19 @@ c3_chart_fn.axis.range = function (range) {
         };
     }
 };
+c3_chart_fn.axisX = function (value){
+    var $$ = this.internal;
+    
+    $$.config.axis_x_show = !!value;
+    $$.axes.x.style("visibility", $$.config.axis_x_show ? 'visible' : 'hidden');
+    
+    $$.redraw();
+};
+c3_chart_fn.axisY = function (value){
+    var $$ = this.internal;
+    
+    $$.config.axis_y_show = !!value;
+    $$.axes.y.style("visibility", $$.config.axis_y_show ? 'visible' : 'hidden');
+    
+    $$.redraw();
+};