This might not be the answer you are looking for, but it is the only way I can get it to show multiple columns on one X-value; by adding each point to a separate Series.

chart1.Series.Add("series1");
chart1.Series["series1"].Points.AddXY("31.5 Hz", 52);
chart1.Series.Add("series2");
chart1.Series["series2"].Points.AddXY("31.5 Hz", 58);
chart1.Series.Add("series3");
chart1.Series["series3"].Points.AddXY("31.5 Hz", 67);