It seems to me that in APEX 4.0.2.00.07 (and probably entire 4.0, but can't check at the moment) there's a bug with adding markers to your (bar/column) chart. For a Pie chart it works fine.
If you edit your Chart Attributes you can define a Marker in the Display Settings.
- add another Series in the chart of type Marker, but this is not my preferred option because you lose performance as it will need to run the query twice.
When you run the page after you changed the XML, the chart with the marker looks like this (see the cross on top of the bar):