For multiple select, expose an array of values.

Author: mbostock

src/generators/input.js

@@ -17,6 +17,7 @@ function valueof(input) {
     case "date": return input.valueAsDate;
     case "checkbox": return input.checked;
     case "file": return input.multiple ? input.files : input.files[0];
+    case "select-multiple": return Array.from(input.selectedOptions, o => o.value);
     default: return input.value;
   }
 }