The observed behavior is due to different number of data points. Please keep in mind by design categoryAxis.labels in grouped chart are created depending on the first group. Also the categorical charts (area, line, bar, column) require a matching set of data points - the value can be null but the record needs to be presented in the data (this is because series.data is a simple array). Hence in order to get the expected result you should add the missing points into the Group2. We realize this is not the most perfect solution and will do our best to provide a better approach for future releases.
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