Insert multilevel categories in barchart uisng OpenXml SKD 2.5 in .net

I want to insert multi-level category in bar chart. I am able to append the single level category using below code which basically add 2 series with a single level category having points "Point 1", "Point 2", "Point 3", "Point 4". For reference here is the link for the code.

 internal StringReference AddStringReference(string formula, params string[] values)
        {
            var strRef = new StringReference();

            strRef.Formula = new Drawing.Charts.Formula() { Text = formula };
            strRef.StringCache = new StringCache();
            strRef.StringCache.PointCount = new PointCount() { Val = new UInt32Value(Convert.ToUInt32(values.Length)) };

            uint index = 0;
            foreach (var value in values)
            {
                strRef.StringCache.AppendChild(new StringPoint { Index = new UInt32Value(index) })
                    .Append(new NumericValue(value));
                index++;
            }

            return strRef;
        }

To add a multi level category we have a class called MultiLevelStringReference. Below is my code for getting the result. But it is not working.

 internal MultiLevelStringReference AddMultiLevelStringReference(string formula, params string[] values)
        {
            var strRef = new MultiLevelStringReference();

            strRef.Formula = new Drawing.Charts.Formula() { Text = "Sheet1!$A2$2:$B$9"}; // for multi-level data 
            strRef.MultiLevelStringCache = new StringCache();
            strRef.MultiLevelStringCache .PointCount = new PointCount() { Val = new UInt32Value(Convert.ToUInt32(values.Length)) };

            uint index = 0;
            foreach (var value in values)
            {
                strRef.MultiLevelStringCache.AppendChild(new StringPoint { Index = new UInt32Value(index) })
                    .Append(new NumericValue(value));
                index++;
            }

            return strRef;
        }
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