I'm a longtime user of a shareware called Overdisk. It has some very good features but is not open source and is not improving itself. You might want to look at it to see if there are some features HDGraph could implement.
Ideas I especially prefer in Overdisk:
show a conventional tree view on left to orient current location on hard drive
Don't show tiny directories, just the big things
arrange directories in decreasing order of size
color directories in decreasing size getting paler as the get smaller
click on a directory to move it to the center, showing decendants around it, with a "go up 1" button on menu
Contact ionfo for Overdisk:
OverDisk was written by Elias Fotinis.
For any bugs, comments, or suggestions, please email me at firstname.lastname@example.org.
You can also check out my website for updates and other free stuff at users.forthnet.gr/pat/efotinis/
(OverDisk page: users.forthnet.gr/pat/efotinis/programs/overdisk.html).