diff --git a/.todo b/.todo
index 6cb1134..a89f511 100644
--- a/.todo
+++ b/.todo
@@ -1,4 +1,3 @@
-document 3D coordinate system
 add audio capabilities
 add fillCircle()
 add gradient functionality
diff --git a/doc/index.html b/doc/index.html
index 59493b6..5fb2f89 100644
--- a/doc/index.html
+++ b/doc/index.html
@@ -26,6 +26,7 @@ The library functions are documented below.
         <ul>
             <li><a href="#invocation">Invocation</a></li>
             <li><a href="#colors">Colors</a></li>
+            <li><a href="#coordinates">Coordinate Systems</a></li>
         </ul>
     </li>
     <li>
@@ -69,6 +70,21 @@ consist of red, green, and blue components.
 Each component is a real number that can range from 0.0 to 1.0.
 </p>
 
+<a name="coordinates" />
+<h3>Coordinate Systems</h3>
+
+<p>
+Anaglym uses the OpenGL right-handed coordinate system for its 3D coordinates.
+The default orientation is such that the XY plane is horizontal (the
+"ground" plane).
+The Z axis extends positively into the "air" above the XY plane, and the
+default gravity vector pulls along the negative Z axis.
+</p>
+<p>
+FIXME: 2D coordinates
+</p>
+
+
 <hr />
 
 <a name="ag" />
@@ -208,7 +224,7 @@ of the circle.
 
 <a name="ag_drawImage" />
 <h3>drawImage</h3>
-<p><tt>ag.drawImage(width, height, x, y, texture, [, rot])</tt></p>
+<p><tt>ag.drawImage(width, height, x, y, texture [, rot])</tt></p>
 <p>
 This function tells the engine to draw an image to the screen.
 <tt>width</tt> and <tt>height</tt> specify the size of the image,
diff --git a/tests/managed_objects.lua b/tests/managed_objects.lua
index cee29d8..c0e9ca3 100644
--- a/tests/managed_objects.lua
+++ b/tests/managed_objects.lua
@@ -11,6 +11,8 @@ function init_event()
         ag.exit()
     end
     ag.setCursorVisible(true)
+    pt_x = -1
+    pt_y = -1
 end
 
 function init_obj(obj)
@@ -66,5 +68,14 @@ function mousebutton_down_event(button, x, y)
                 obj:setColor(0, 0, 1)
             end
         end
+    elseif (button == 3) then
+        pt_x = x
+        pt_y = y
+    end
+end
+
+function update_overlay_event(width, height)
+    if (pt_x >= 0 and pt_y >= 0) then
+        ag.drawCircle(0, 1, 0, pt_x, pt_y, 5)
     end
 end