How to add the Chart sheet in Excel using C#

Try this (UNTESTED)

Excel.ChartObject myChart = (Excel.ChartObject)charts.Add(10, 70, 250, 250);

instead of

Excel.ChartObject myChart = (Excel.ChartObject)workbook.Charts[2];

and then once your chart is created, move it to a chart sheet using this code

chart.Location(XlChartLocation.xlLocationAsNewSheet, Type.Missing);