Adding isNumber internal function

c3.js

@@ -5532,6 +5532,9 @@
     var isValue = c3_chart_internal_fn.isValue = function (v) {
             return v || v === 0;
         },
+        isNumber = c3_chart_internal_fn.isNumber = function (v) {
+            return !isNaN(parseFloat(v)) && isFinite(v);
+        },
         isFunction = c3_chart_internal_fn.isFunction = function (o) {
             return typeof o === 'function';
         },