I pushed the patch to require resolution today, expect this to hit the general public with libinput 1.6. If your graphics tablet does not provide axis resolution we will need to add a hwdb entry. Please file a bug in systemd and CC me on it (@whot).
How do you know if your device has resolution? Run sudo evemu-describe against the device node and look for the ABS_X/ABS_Y entries:
# Event code 0 (ABS_X) # Value 2550 # Min 0 # Max 3968 # Fuzz 0 # Flat 0 # Resolution 13 # Event code 1 (ABS_Y) # Value 1323 # Min 0 # Max 2240 # Fuzz 0 # Flat 0 # Resolution 13if the Resolution value is 0 you'll need a hwdb entry or your tablet will stop working in libinput 1.6. You can file the bug now and we can get it fixed, that way it'll be in place once 1.6 comes out.