The solution were to change the YValueType for the Series myChart1.Series[1].YValueType = ChartValueType.Int32; to myChart1.Series[1].YValueType = ChartValueType.Double;.

Now both minimum and maximum YValues on the chart, corresponds to the values set in the code.

myChart1.ChartAreas[0].AxisY.Minimum = 9.3;
myChart1.ChartAreas[0].AxisY.Maximum = 10.3;