Set the X axis in subchart to visible or invisible

I'm totally new in the github and all this stuff. Sorry if I do something wrong. 

I came across c3 for two days and found it suitable for me. Thanks for that!

But there are some points which I would like to improve. Here I have added two lines to the c3.js
and with following options the X axis could be set to hide
```
...
axis: {
  x: {
    show: false
  }
}
...
```
http://jsfiddle.net/vladsu/89szjf3r/3/ 

Best wishes,
Vlad

c3.js

@@ -1077,6 +1077,7 @@
             subchart_show: false,
             subchart_size_height: 60,
             subchart_onbrush: function () {},
+            subchart_axis_x_show: true,
             // color
             color_pattern: [],
             color_threshold: {},
@@ -5264,7 +5265,8 @@
         $$.axes.subx = context.append("g")
             .attr("class", CLASS.axisX)
             .attr("transform", $$.getTranslate('subx'))
-            .attr("clip-path", config.axis_rotated ? "" : $$.clipPathForXAxis);
+            .attr("clip-path", config.axis_rotated ? "" : $$.clipPathForXAxis)
+            .style("visibility", config.subchart_axis_x_show ? 'visible' : 'hidden');
     };
     c3_chart_internal_fn.updateTargetsForSubchart = function (targets) {
         var $$ = this, context = $$.context, config = $$.config,