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:toStringForValueAxis:.

For example, use the following code:

- (NSString *)valueAxisDataSourceDouble:(double)value toStringForValueAxis:(NChartValueAxis *)axis
{
    return [NSString stringWithFormat:@"value %f", 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;