var model = new PlotModel();
model.MouseDown += (s, e) =>
{
	if (e.ChangedButton == OxyMouseButton.Left)
	{
		foreach (var hit in model.HitTest(new HitTestArguments(e.Position, 0)))
		{
			if (hit.Item is BarItem barItem)
			{
				// change the color of the clicked Bar Item to
				barItem.Color = OxyColors.Red;
				model.InvalidatePlot(false);
			}
		}
	}
};
var s1 = new BarSeries { Title = "Series 1", IsStacked = true, StrokeColor = OxyColors.Black, StrokeThickness = 1, StackGroup = "3" };
int i = 0;
s1.Items.Add(new BarItem { Value = 25, CategoryIndex = i++ });
s1.Items.Add(new BarItem { Value = double.NaN, CategoryIndex = i++ }); // add one invalid item
s1.Items.Add(new BarItem { Value = 18, CategoryIndex = i++ });
s1.Items.Add(new BarItem { Value = 40, CategoryIndex = i++ });
model.Series.Add(s1);