//percent
chart1.ChartAreas[yourCharArea].AxisY.LabelStyle.Format = "###0\\\\%";
//or
chart1.ChartAreas[yourCharArea].AxisY.LabelStyle.Format = "###0.0\\\\%";

//thousand
chart.ChartAreas[0].AxisY.LabelStyle.Format = "#,##0,k"

//million
chart.ChartAreas[0].AxisY.LabelStyle.Format = "#,##0,,M";