You can change the format by overriding the double-to-string conversion of the needed axis. This can be achieved by implementing the corresponding method of value axis data source valueAxisDataSourceDouble(value,toStringForValueAxis).

For example, use the following code:

func valueAxisDataSourceDouble(_ value: Double, toStringFor axis: NChartValueAxis!) -> String! {
    return "value \(value)"
}

Do not forget to set the data source for the axis you want to customize, for example:

m_view.chart.cartesianSystem.yAxis.dataSource = self