Adding isNumber internal function

c3.js

@@ -5530,8 +5530,11 @@
     };
 
     var isValue = c3_chart_internal_fn.isValue = function (v) {
-        return v || v === 0;
-    },
+            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';
         },