run the attached example (chart-change-type.zul) and toggle the chart type:

with every update the chart data is appended again (below extracts from the AJAX responses from the server to the browser)

initial rendering:

plotData:{"chart":{},"series":[{"data":[4,7],"name":"Claim"}]

after changing type to "bar":

plotData:{"chart":{"type":"bar"},"series":[{"data":[4,7,4,7],"name":"Claim"}]

after changing type back to "line":

plotData:{"chart":{"type":"line"},"series":[{"data":[4,7,4,7,4,7],"name":"Claim"}]

WORKAROUND:

// WORKAROUND 1 - reset the same model
ChartsModel oldModel = mychart.getModel();
mychart.setModel(new DefaultCategoryModel());
mychart.setModel(oldModel);

// WORKAROUND 2 - replace with a new model
mychart.setModel(createModel());