When using dual or multiple color axes, this number defines which colorAxis the particular series is connected to. It refers to either the {@link #colorAxis.id|axis id} or the index of the axis in the colorAxis array, with 0 being the first. Set this option to false to prevent a series from connecting to the default color axis.
Since v7.2.0 the option can also be an axis id or an axis index instead of a boolean flag.