The graph range used to be reset to default whenever all functions were
modified. As we no longer want to reset the range without the user's
input, we do not need to track whether the functions changed at all.
/!\ As of this commit, there is no longer a way to restore the default
zoom, until a new automatic zoom button is added.
Change-Id: Ie74e8fd61e13055fa6ce2b2d1e883182d4ecffce