Try this:
public class Data
{
public DateTime Date { get; set; }
public int Value { get; set; }
}
...
List<Data> data = new List<Data>();
for (int mon = 1; mon < 13; mon++)
{
data.Add(new Data() { Date = new DateTime(2018, mon, 1), Value = mon });
}
chart1.DataSource = data;
chart1.Series[0].XValueMember = "Date";
chart1.Series[0].YValueMembers = "Value";
chart1.ChartAreas[0].AxisX.IntervalType = System.Windows.Forms.DataVisualization.Charting.DateTimeIntervalType.Months;
chart1.ChartAreas[0].AxisX.LabelStyle.Format = "MMM";
chart1.ChartAreas[0].AxisX.IntervalAutoMode = System.Windows.Forms.DataVisualization.Charting.IntervalAutoMode.VariableCount;
chart1.ChartAreas[0].AxisX.Interval = 1;
chart1.DataBind();