private void theChart_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
var chart = (XamDataChart)sender;
var s = chart.Series.FirstOrDefault();
if (s == null)
return;

var position = e.GetPosition(s);
var xAxis = theChart.Axes.OfType<CategoryXAxis>().First();
var yAxis = theChart.Axes.OfType<NumericYAxis>().First();

var viewport = new Rect(0, 0, xAxis.ActualWidth, yAxis.ActualHeight);
var window = s.SeriesViewer.WindowRect;

var scalerParams = new ScalerParams(window, viewport, false);

var unscaledX = xAxis.GetUnscaledValue(position.X, scalerParams);
var unscaledY = yAxis.GetUnscaledValue(position.Y, scalerParams);

var index = (int)Math.Floor(unscaledX);

var idx = 0;
var iEnumerable = s.ItemsSource.GetEnumerator();
var item = default(ChartData);
while (iEnumerable.MoveNext())
{
if (idx == index)
{
item = (ChartData)iEnumerable.Current;
break;
}

idx++;
}

Console.WriteLine(item.Date);
}
}

public class ChartData
{
public DateTime Date { get; set; }
public double Value1 { get; set; }
public double Value2 { get; set; }
public double Value3 { get; set; }

public static ObservableCollection<ChartData> GenerateData(int count)
{
ObservableCollection<ChartData> data = new ObservableCollection<ChartData>();

Random r = new Random();

ChartData data0 = new ChartData
{
Date = DateTime.Now,
Value1 = r.Next(20, 40),
Value2 = r.Next(20, 40),
Value3 = r.Next(20, 40),
};

data.Add(data0);

for (int i = 1; i < count; i++)
{
ChartData dat = new ChartData
{
Date = DateTime.Now.AddDays(i),
};

if (r.NextDouble() > .5)
{
dat.Value1 = data[i - 1].Value1 + r.NextDouble();
dat.Value2 = data[i - 1].Value2 + r.NextDouble();
dat.Value3 = data[i - 1].Value3 + r.NextDouble();
}
else
{
dat.Value1 = data[i - 1].Value1 - r.NextDouble();
dat.Value2 = data[i - 1].Value2 - r.NextDouble();
dat.Value3 = data[i - 1].Value3 - r.NextDouble();
}

data.Add(dat);
}

return data;
}
}