running the attached example renders the chart correctly on the initial request.

when updating the chart model series with the same data the 2nd series renders at the wrong position causing graphics errors.

This happens when a series does not contain values for all categories

• values missing at the beginning cause the data to shift to the left
• values missing at the end cause a visual glitch at the right end of the incomplete series

When swapping the code in lines 89/90 of ChartsTestVM making the first series incomplete causes the same issue with another sideeffect, that shows a related issue, that the categories are not sorted, even if Comparable might suggest that. (org.zkoss.chart.model.CategoryModel.setValue(Comparable<?>, Comparable<?>, Number))
here a related topic topic about positioning category values
http://stackoverflow.com/questions/15483416/sort-of-series-values-for-xaxis

Current workaround is set the categories of a series without gaps, and in the same order as all other series.