orderIndex

This index is used to determine the position of the series entry in the legend:

 dataSeries: [
    {
        seriesType: 'line',
        orderIndex: 3,
        data: [123.4, 71.5, 106.4, 144.0, 176.0, 135.6, 148.5, 54.4]
    }
]

This setting is demonstrated in the following example.