A way to reproduce this bug on a phone with higher resolution is to try compare mode with the phone in landscape orientation.
On a newer phone, all the 6 available data sets in my area can fit in upright orientation, but in landscape orientation only 3 graphs are displayed at the time, and hence there is a need for both scrolling (i. e. the up/down component of a swiping motion on the screen, moving the whole image/page), and
to move the line left to tight (i.e. by swiping horizontally on the screen).
And then this bug becomes apparent: