* data model priority: if there is data model passed
* through data attribute use it. Otherwise nested point
* tags are expected.
*/
VisitSeries seriesCallback = new VisitSeries(s.getType());
s.visitTree(VisitContext.createVisitContext(FacesContext
.getCurrentInstance()), seriesCallback);
model = seriesCallback.getModel();
// if series has no data create empty model
if (model == null) {