Fix incorrect (?) flip=True within plot_file() and plot_lines()

[doersino]

When

1. plotting a file via plot_file() or
2. a set of lines via plot_lines(),

my drawings come out flipped along the x-axis. Taking a look at the code, I noticed that

1. plot_file() calls plot_lines() with flip=True, which seems wrong but the removal of which didn't fix my problems on its own, and
2. plot_lines() calls rotate_and_scale_lines() with flip=True. Instead supplying flip=flip, in conjunction with the previous point, fixes both of my problems.

According to git blame, these two occurrences of flip=True have been present for quite a while. Given this and the lack of issues or pull requests relating to the problems I encountered, I'm not sure if this is actually wrong or if something on my side is set up incorrectly.

[doersino]

Plus, I think that

if flip ^ rotate:

in rotate_and_scale_lines() must be replaced with just

if flip:

but the same caveats as above apply. Maybe it's just something with my setup.