id should be a string not a length-1 array

- this didn't lead to any bugs, as
  var a = ['x' + 'y']
  var b = {}
  b[a] = 1
  // => {'xy': 1}

Author: etpinard

src/traces/splom/defaults.js

@@ -89,7 +89,7 @@ function handleAxisDefaults(traceIn, traceOut, layout, coerce) {
     // fill in splom subplot keys
     for(i = 0; i < axLength; i++) {
         for(j = 0; j < axLength; j++) {
-            var id = [xaxes[i] + yaxes[j]];
+            var id = xaxes[i] + yaxes[j];
 
             if(i > j && showUpper) {
                 layout._splomSubplots[id] = 1;