Change the XAxis.AxisTickOrigin property after the BuillGraph call. That will place the label in the center of the first bar.
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this.AddCategoryItem(27, "Holes");
this.AddCategoryItem(8, "Stains");
this.MainTitle.TextString = "Pareto Diagram of Defects";
this.BuildChart();
this.XAxis.AxisTickOrigin = 0.5;