When you define each item in your series it is placed in the legend in the defined order.  You can modify the ordering using the legendItem  and setting order property.

series: [
    {
        legendItem : {
            visible : false
        },
values: [35,42,67,89,25,34,67,85]
    },
    {
        text: "Second Series",
values: [2,4,6,8,10,8,6,4]
    },
    {
        text: "Series #3",
legendItem: {
   order : 1
},
values: [21,18,15,12,15,18,21,24]
    }
]

Example: https://demos.zingchart.com/view/IVG5BHU8

Documentation

Legend Tutorial
series JSON Syntax
legendItem JSON Syntax

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