add wait_for_output(cell, index)

so you can wait for at least n outputs

IPython/html/tests/casperjs/util.js

@@ -57,15 +57,18 b' casper.delete_current_notebook = function () {'
     });
 };
 
-// wait for output in a given cell
-casper.wait_for_output = function (cell_num) {
-    this.waitFor(function (c) {
-        return this.evaluate(function get_output(c) {
-            var cell = IPython.notebook.get_cell(c);
-            return cell.output_area.outputs.length != 0;
-        },
-        // pass parameter from the test suite js to the browser code js
-        {c : cell_num});
+// wait for the nth output in a given cell
+casper.wait_for_output = function (cell_num, out_num) {
+    out_num = out_num || 0;
+    this.then(function() {
+        this.waitFor(function (c, o) {
+            return this.evaluate(function get_output(c, o) {
+                var cell = IPython.notebook.get_cell(c);
+                return cell.output_area.outputs.length > o;
+            },
+            // pass parameter from the test suite js to the browser code js
+            {c : cell_num, o : out_num});
+        });
     },
     function then() { },
     function timeout() {