This is a pretty common mistake in JFreeChart. Each category in your chart ('abc', 'xyz' etc) has a value for each of the 6 series ('q1', 'q2' ... 'q6'), so there are 36 data items in all. But 30 of those data items are null, because you didn't specify them. JFreeChart leaves a space where the bar would appear, when the data value is null.
If you really have only six data values, you should have one series with 6 categories, or one category and six series.
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