To customize axis labels, you override the ResolveLabelForAxis method in a class that derives from IGChartViewDelegate. The code below populates an array of custom data objects and customizes both the x and y axis labels of a column series chart.

namespace ChartCustomDataAxisLabels
{
    public class GraphObject : NSObject
    {
        [Export("Territory")]
        public String Territory {get; set;}
        [Export("Cost")]
        public Double Cost {get; set;}

        public GraphObject() {}
    }

    public class ChartViewDelegate : IGChartViewDelegate
    {
        public override string ResolveLabelForAxis (IGChartView chartView, IGAxis axis, NSObject item)
        {
            if (axis.Key.Equals("xAxis"))
            {
                return ((IGCategoryPoint)item).Label;
            }
            else if (axis.Key.Equals("yAxis"))
            {
                NSNumberFormatter formatter = new NSNumberFormatter();
                formatter.NumberStyle = NSNumberFormatterStyle.Currency;
                return formatter.StringFromNumber((NSNumber)item);
            }

            return String.Empty;
        }
    }

    public partial class ChartCustomDataAxisLabelsViewController : UIViewController
    {
        IGChartView _chart = new IGChartView();
        List _data = new List();
        IGCategorySeriesDataSourceHelper _source = new IGCategorySeriesDataSourceHelper();

        public ChartCustomDataAxisLabelsViewController () {}
        
        public override void ViewDidLoad ()
        {
            base.ViewDidLoad ();

            this.generateData();
            _source.Data = _data.ToArray();
            _source.ValuePath = @"Cost";
            _source.LabelPath = @"Territory";

            RectangleF chartRect = this.View.Frame;
            chartRect.Inflate(-30.0f, -30.0f);
            _chart.Frame = chartRect;
            _chart.AutoresizingMask = UIViewAutoresizing.FlexibleHeight|UIViewAutoresizing.FlexibleWidth;
            _chart.Theme = IGChartGradientThemes.IGThemeDark();
            _chart.Delegate = new ChartViewDelegate();
            this.View.Add(_chart);

            _chart.AddSeries(new MonoTouch.ObjCRuntime.Class("IGColumnSeries"), "series", _source, "xAxis", "yAxis");
        }

        private void generateData()
        {
            String[] territories = new string[] {@"Canada", @"Finland", @"Holland", @"Japan", @"USA"};
            
            for (int i = 0; i < 5; i++)
            {
                GraphObject graphObject = new GraphObject();
                graphObject.Territory = territories[i];
                graphObject.Cost = new Random(i).Next(100000);
                _data.Add(graphObject);
            }
        }
    }
}