If I'm not mistaken, you are looking for a graph such as a stack, you need to add a third series to indicate the difference, try to follow
Series Series1 = new Series();
Series Series2 = new Series();
Series Series3 = new Series();
Series1.ChartType = SeriesChartType.Column;
Series2.ChartType = SeriesChartType.StackedColumn;
Series3.ChartType = SeriesChartType.StackedColumn;
int series1Data=30;
int series2Data=40;
int series3Data=series2Data-series1Data;
series2Data=series1Data;
Series1.Points.Add(series1Data);
Series2.Points.Add(series2Data);
Series3.Points.Add(series3Data);
Chart1.Series.Add(Series1);
Chart1.Series.Add(Series2);
Chart1.Series.Add(Series3);
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