Charts - Address issue in which column/bar graphs with missing data and groupMarkers set to true

Proposed fix for: https://github.com/yui/yui3/issues/1392

Need to test:
- [ ] ie6
- [ ] ie7
- [ ] ie8
- [ ] ie9
- [ ] ie10
- [ ] ie11
- [ ] mac safari
- [ ] ios safari
- [ ] android 2.3.4
- [ ] android 4.1.1
- [ ] firefox
- [ ] chrome