Mate, this is probably not an MT-4 Related thing, but more to do with your computer.
You click on a chart TO SELECT IT
you click on the TOOL to SELECT IT (Horizontal Line) from the tool bar
in the background there is a technical process that associates these 2 things and assumes you want to put the line
on the chart
you then click on the chart
THE LINE IS PLACED
going off that logic that the computer uses to place a line on a chart
or more specifically .... to change the colour of a set number of pixels on the Horizontal scale of the Screen Matrix
to change 2 charts of different instruments
YOU WOULD NEED TO SELECT THEM
My question would be
do you have a touch screen ?
do you have a stylus and touchpad or something to that effect ?
if so have you calibrated them ?
if MT-4 is doing this, you could try backing everything up and reinstalling it
or better still
keep the current installation and install another instance of MT-4 and see if IT does it as well
if it does
then it's your computer
if it doesn't then it's the first MT-4 installation
and since the 2nd doesnt do it
you could just reinstall the first, knowing that it will work
but to me, i would check , Mouse and keyboard and input device settings
Uninstall and reinstall from device manager, Your keyboard and mouse
restart your computer.
see how you go
try it again.