Inverted Horizontal scrolling ubuntu 18.04

I also encountered this issue upon upgrading to 18.04, this was my solution:

Use xinput list to find the device id of your touchpad.

Use xinput list-props yourdeviceid. This will produce a long list of all the properties you can edit for that device. We're interested in a property to do with scrolling distance, on my system this is Synaptics Scrolling Distance (283). It should have two values, on my system (with natural scrolling enabled) these were -115, 115 (vertical distance, horizontal distance). Note the value in the parentheses, in my case 283, it's how we'll identify the property to change it.

Use xinput set-prop yourdeviceid 283 -115, -115, replacing 283 and the scrolling distance values with whatever is appropriate. (The change is to make both values negative, which gives the desired result of "natural" scrolling.)

Notes:
This setting will not persist across system restarts, which is an issue all on its own. I use a .xsessionrc file in my home directory to execute the xinput command on startup.
This will probably not work in 17.10, since Wayland does strange things to xinput.

See this:

https://help.ubuntu.com/community/SynapticsTouchpad

Use the following commands to set the amount and direction of natural scrolling (plus or minus values change direction):

synclient HorizScrollDelta=-100
synclient VertScrollDelta=-100

You can put this command in your autostart script to run it when you log in.

Here's a little script that does it for you

export id=$(xinput list | grep -i touchpad | awk -F"=" '{ print $2 }' | awk '{ print $1 }')
xinput list-props "${id}" | grep "Synaptics Scrolling Distance" | sed 's/[^0-9 \t-]//g' | while read a b c;
do
  echo "${a} ${b} $((${c}*-1))";
  xinput set-prop "${id}" "${a}" "${b}" "$((${c}*-1))"
done

I had this problem for a long time. I recently updated to Ubuntu 19.10 and it was still there, so I poked around again for solutions. It appears that this is due to having the (apparently obsolete) "synaptics" input driver installed either instead of or in addition to the "libinput" driver. To correct this, run these two commands:

sudo apt install xserver-xorg-input-libinput

sudo apt remove xserver-xorg-input-synaptics

After that, restart X11 by logging out and back in or restarting the system. I'm not quite sure if it started working for me immediately or if I had to toggle the Natural Scrolling setting on and off in the Mouse & Touchpad screen in Gnome settings -- I know that I had to adjust the Touchpad Speed since that was waaay off.

Solution here from Gnome's GitLab issues: https://gitlab.gnome.org/GNOME/gnome-shell/issues/1808#note_629824

Delete the xserver-xorg-input-libinput by sudo apt remove xserver-xorg-input-libinput.

Then create the file /usr/share/X11/xorg.conf.d/20-natural-scrolling-mouses.conf with the following content:

Section "InputClass"  
    Identifier "Natural Scrolling Mouses"  
    MatchIsPointer "on"  
    MatchIsTouchpad "off"  
    MatchDevicePath "/dev/input/event*"  
    Option "VertScrollDelta" "-1"  
    Option "HorizScrollDelta" "-1"  
    Option "DialDelta" "-1"  
EndSection

After rebooting, the scrolling issue was solved for me.

fwiw, I wrote a script based on these answers that will figure out all the input IDs and whatnot for you. Bonus: you can choose "NATURAL" or "REVERSE" scrolling by passing either as a positional arg to the script

#!/usr/bin/env bash

##############################################################
# DESCRIPTION:  Fixes touchpad natural scrolling for horizontal
##############################################################
set -x
set -e

TYPE="${1:-NATURAL}"

TOUCHPAD_ID=$( xinput list | grep Touch | sed 's/\s\s*\[.*//;' | awk -F'=' '{print $NF}' )

PROPS=$( xinput list-props ${TOUCHPAD_ID} | \
    grep "Synaptics Scrolling Distance" | \
    sed 's/[^0-9][^0-9]*/ /g' | awk '{print $1" "$2" "$3" "}'
    )

PROP1=$( echo "$PROPS" | awk '{print $1}' )
PROP2=$( echo "$PROPS" | awk '{print $2}' )
PROP3=$( echo "$PROPS" | awk '{print $3}' )

if [[ ${TYPE} == "NATURAL" ]]; then
    xinput set-prop ${TOUCHPAD_ID} ${PROP1} -${PROP2} -${PROP3}
else
    xinput set-prop ${TOUCHPAD_ID} ${PROP1} ${PROP2} ${PROP3}
fi

In case this is still an issue, you can also change your setting in: Setting | Devices | Mouse and Touchpad. There the option "natural scrolling" will enable changes in the scrolling behavior.