plotly#189 adding missing docs and reducing assumptions

Author: monfera

src/plots/cartesian/ordered_categories.js

@@ -13,14 +13,20 @@ var d3 = require('d3');
 
 
 /**
- * TODO add documentation
+ * This pure function returns the ordered categories for specified axisLetter, categorymode, categorylist and data.
+ *
+ * If categorymode is 'array', the result is a fresh copy of categorylist, or if unspecified, an empty array.
+ *
+ * If categorymode is 'category ascending' or 'category descending', the result is an array of ascending or descending
+ * order of the unique categories encountered in the data for specified axisLetter.
+ *
  */
 module.exports = function orderedCategories(axisLetter, categorymode, categorylist, data) {
 
     return categorymode === 'array' ?
 
         // just return a copy of the specified array ...
-        categorylist.slice() :
+        (Array.isArray(categorylist) ? categorylist : []).slice() :
 
         // ... or take the union of all encountered tick keys and sort them as specified
         // (could be simplified with lodash-fp or ramda)