The reported behavior is caused by the fact that the chart is allocating equal space in each category slot (the distance between two adjacent ticks on the axis) for each type of bar series. For example, when you define stacked and clustered series the category slot will be split in half and one part will be used for the stacked series and the other for the clustered ones located in the slot.
In your case you have 2 clustered series and 1 stacked, which means the clustered series will occupy the left side and the stacked series will occupy the right side. For example, if the entire slot's width is 100 pixels, the clustered series will use 50px (so 25 pixels per bar). On the other hand the stacked series will use the 50px (so 50 pixels per bar).
Please let me know if you have any further questions.
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