Add an error message if the graph placeholder isn't found.

morris.coffee

@@ -14,6 +14,10 @@ class Morris.Line
       @el = $ document.getElementById(options.element)
     else
       @el = $ options.element
+
+    if @el == null || @el.length == 0
+      throw new Error("Graph placeholder not found.")
+
     @options = $.extend {}, @defaults, options
     # backwards compatibility for units -> postUnits
     if typeof @options.units is 'string'

morris.js

@@ -29,6 +29,9 @@
       } else {
         this.el = $(options.element);
       }
+      if (this.el === null || this.el.length === 0) {
+        throw new Error("Graph placeholder not found.");
+      }
       this.options = $.extend({}, this.defaults, options);
       if (typeof this.options.units === 'string') {
         this.options.postUnits = options.units;

test/test.html

@@ -24,6 +24,18 @@
         deepEqual(my_data, expected_data);
     });
 
+    test("Nicer error when the placeholder was not found.", function () {
+        var my_data = [{x: 1, y: 1}, {x: 2, y: 2}];
+        raises(function() {
+            Morris.Line({
+                element: "thisplacedoesnotexist",
+                data: my_data,
+                xkey: 'x',
+                ykeys: ['y'],
+                labels: ['dontcare']})
+        }, "Graph placeholder not found.");
+    });
+
     test("Morris.commas", function () {
       // zero
       equal(Morris.commas(0), "0", "commas(0) = 0")