histogram function with actual bucket boundary

 Hi,
Is there anyway to get histogram function with actual bucket boundary? I have a query like this.

SELECT row_size, Count(*) frequency FROM    (
SELECT WIDTH_BUCKET (Nvl(VSize(table_name),0)+1,0,30,3) row_size
  FROM user_tables
) GROUP BY row_size ORDER BY row_size

ROW_SIZE FREQUENCY
       1       110
       2         6

But I would rather the result set to be

ROW_SIZE FREQUENCY
      20       110
      30         6

I know the following can do it.

SELECT    row_size, Count(*) frequency FROM    (
SELECT 10 * Round((Nvl(VSize(table_name),0)+1)/ 20)+20 row_size
  FROM user_tables
) GROUP BY row_size  ORDER BY row_size

However, I would like to have a oracle built-in function to achieve the same. Any input?