Accept arrays for extra argument of Axes.coerceRef

This way we can say that either 'pixel' or 'paper' is OK for axis
references of annotations.

Author: nicholas-esterer

src/components/annotations/defaults.js

@@ -60,7 +60,8 @@ function handleAnnotationDefaults(annIn, annOut, fullLayout) {
         if(showArrow) {
             var arrowPosAttr = 'a' + axLetter;
             // axref, ayref
-            var aaxRef = Axes.coerceRef(annIn, annOut, gdMock, arrowPosAttr, '', 'paper', true);
+            var aaxRef = Axes.coerceRef(annIn, annOut, gdMock, arrowPosAttr, '',
+                                        ['paper','pixel'], true);
 
             // for now the arrow can only be on the same axis or specified as pixels
             // TODO: sometime it might be interesting to allow it to be on *any* axis

src/plots/cartesian/axes.js

@@ -96,7 +96,9 @@ axes.coerceRef = function(containerIn, containerOut, gd, attr, dflt, extraOption
 
     attrDef[refAttr] = {
         valType: 'enumerated',
-        values: axlist.concat(extraOption ? [extraOption] : []),
+        values: axlist.concat(extraOption ?
+            (typeof extraOption === 'string' ? [extraOption] : extraOption)
+            : []),
         dflt: dflt
     };