In such cases you should always open Object Browser in excel macros and and search for what you are looking for

As you can see from object browser, the value for histogram is 118
or 76
in hex. You can use the same in your code directly by defining a constant
Edit-1: The code
Debugging your code below I found a issue
Worksheet sheet1 = Globals.Factory.GetVstoObject(Globals.Sheet1.Application.Worksheets[1]);
//chartTest
Excel.ChartObject myChart = (Excel.ChartObject)sheet1.ChartObjects("chartTest");
myChart.Chart.SetSourceData(sheet1.Range["A1", "A51"]);
myChart.Chart.Type = 118;
What you need to do is assign 118
to ChartType
and not Type
. Below code worked fine for me
Worksheet sheet1 = Globals.Factory.GetVstoObject(Globals.Sheet1.Application.Worksheets[1]);
//chartTest
Excel.ChartObject myChart = (Excel.ChartObject)sheet1.ChartObjects("chartTest");
myChart.Chart.SetSourceData(sheet1.Range["A1", "A51"]);
Excel.XlChartType myType = (Excel.XlChartType)118;
myChart.Chart.ChartType = myType;