The chart control as such doesn't have this logic.
Check the solution of this question.
Hope this helps.
Edit: In the meanwhile, I tried the below code snippet in a sample program with your zoom code. Now the axis values are displayed without decimals.
chart1.ChartAreas[0].AxisX.LabelStyle.Format = "0";
chart1.ChartAreas[0].AxisY.LabelStyle.Format = "0";