We built Crab so we could run SQL queries over the filesystem. It's for Win and Mac, free for personal use, and lets you slice this anyway you want and plot the data how you like. I like the flexibility.

http://etia.co.uk/win/about/

e.g.

  SELECT sum(bytes) FROM files
  GROUP BY extension 
  ORDER BY sum(bytes) DESC;

You can GROUP BY any file attributes