Is there a tool to visualize a filesystem allocation map on Linux?

I had the same question, but there was no appropriate software. I tried to build davl, but did not succeed in that. So I ended up writing my own tool. You can find it here: https://github.com/i-rinat/fragview

Use Ctrl + mouse scroll to change map scale.

There's dav, the "Disk Allocation Viewer" (supports ext2 and ext3; websites dates back to 2005, could be a little bit-rotted...)