Adjust Trackpoint speed on Ubuntu 22.04

 ref: https://askubuntu.com/questions/1432996/adjust-trackpoint-speed-on-lenovo-p70-ubuntu-22-04

Here's what worked for me; your mileage may vary. First, find your device:

$ xinput list
⎡ Virtual core pointer                      id=2    [master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer                id=4    [slave  pointer  (2)]
⎜   ↳ TPPS/2 IBM TrackPoint                     id=12   [slave  pointer  (2)]
⎜   ↳ Synaptics TM3053-004                      id=11   [slave  pointer  (2)]
⎜   ↳ ELAN Touchscreen                          id=9    [slave  pointer  (2)]
...

On my system, I see it as TPPS/2 IBM TrackPoint. Next, find the acceleration property:

$ xinput list-props 'TPPS/2 IBM TrackPoint' | grep Accel
    libinput Accel Speed (342): 0.000000
    libinput Accel Speed Default (343): 0.000000
    libinput Accel Profiles Available (344):    1, 1
    libinput Accel Profile Enabled (345):   1, 0
    libinput Accel Profile Enabled Default (346):   1, 0

Here, I see it as Accel Speed (342), and the 342 is what I need. After some experimentation, I was able to determine that the value, by default 0.000000 is expected to be in the range [-1, 1]. For me, -0.2 ended up being a good speed:

$ xinput set-prop 'TPPS/2 IBM TrackPoint' 342 -0.2