Multiple fields in GroupBy clause in LINQ (where one is computed field)

I have a LINQ query upon which I need to add two fields as group by clauses. While I can easily group by with as many column fields but the problem is occurring when one of the fields is a calculated field. I can't seem to be able to get my head around on how to add the second attribute in this case

var values = intermediateValues
            //.GroupBy(x => new {x.Rate, x.ExpiryDate })
            .GroupBy(r => new { Rate = ((int)(r.Rate / BucketSize)) * BucketSize })
            .Select(y => new FXOptionScatterplotValue
            {
                Volume = y.Sum(z => z.TransactionType == "TERMINATION" ? -z.Volume : z.Volume),
                Rate = y.Key.Rate,
                ExpiryDate = y.Key.ExpiryDate,
                Count = y.Count()
            }).ToArray();

In the above code sample I would like to have ExpiryDate added to my existing GroupBy clause which has a computed field of Rate already there. The code looks like this in VS editor