Still, on Yosemite the HP driver installer presents you a strange message "Unsupported operating system, supported versions are 10.6 and above"! Must be joking.
By the way, this reminds me of windows 9 thingie.
Luckily, there is a way to install the software: thanks to Justin's blog, one can easily force installer in belief it's about to install on 10.9 (that's what I did), which is "above" 10.6 (while 10.10.1 is seemingly not).
Word of caution: OSX knows to go hayway during this operation (Finder did it for me). So, do not reboot with changed plist, and quit any application running! What I did was prepare to launch installer (mounted the dmg), made a copy of the plist file, edited the plist by setting version to "10.9", performed the install, and immediately mv the "original" back into place (to reset the version to actual one). During install installer restarted Finder, and it went nuts (some icons did disappear). Then I verified that HP Scan still works, and did a reboot (as the plist file change did trigger some strange things).
HP Scan app still works and handles the scanner just fine on Yosemite.