Please add this code to the code of your previous question.
Chart.chart_main.ChartAreas["BoxPlot"].AxisX.LabelStyle.Angle = -90;
for (int i = 0; i < xValue.Count; i++)
{
Chart.chart_main.ChartAreas["BoxPlot"].AxisX.CustomLabels.Add((i + 1)* 2d, 0.0, xValue[i]);
}