ok i find out the problem. turn out that you set a size for points that has value 0 with this line

minPointLength: 3

and the values with 10 15 20 are at yAxis starting at 0 but also the point with 0 value are starting at 0 and by you setting minPointLength the points with 0 value are bigger than the others and are on top of the others, so the stacked values are 0-15 and the 0 has extra size that is not stacked. did i explained myself correctly?

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