Comment for TRACE_TO_AXIS

Author: VeraZab

src/lib/constants.js

@@ -49,6 +49,8 @@ export const ESCAPE_KEY = 'Escape';
 export const COMMAND_KEY = 'Meta';
 export const CONTROL_KEY = 'Control';
 
+// matches gd._fullLayout._subplots categories except for xaxis & yaxis which
+// are in fact cartesian types
 export const TRACE_TO_AXIS = {
   cartesian: [
     'scatter',