Draw average line before expect line and change their colors
Refix high CPU usage issue

After refactoring out Widget.needRedraw(),
the spans' equality check became malfunctioned
due to Generator comparision.
Changed Record.spans() to return a list instead of
a Generator to fix this.
Refix high CPU usage issue

Merge branch 'dev'
Fix statistics toggle issue

Turn off statistics (press 's' again) is malfunctioned
before because self.update() is only called on toggle-on,
change to call self.update() both in toggle-on and toggle-off
to fix.
Fix statistics toggle issue

Merge branch 'dev'
Add missed imports

Merge 74a354b is wrong/incomplete because it omits
the required imports in main.py of dev.