no need to cast string to string

Co-Authored-By: alexcjohnson <johnson.alex.c@gmail.com>

Author: archmoj

src/plots/cartesian/axis_defaults.js

@@ -225,6 +225,6 @@ var dayStrToNum = {
 function indexOfDay(v) {
     if(typeof v !== 'string') return;
     return dayStrToNum[
-        String(v).substr(0, 3).toLowerCase()
+        v.substr(0, 3).toLowerCase()
     ];
 }