You have four values all for the same moment of time. Here are two possible solutions. First, if you want them as two different pairs you should join them like this:

@data = (
    ["2010/03/29 14:00:00", "2010/03/29 14:00:00"],
    [13, 15],
    [20, 21],
);
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Second, if they are really for the same time, maybe they should be like this:

@data = (["2010/03/29 14:00:00"], [13], [15], [20], [21],);
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