  Sine illusion happens when the more quickly changing pairs of lines lead to
bigger underestimates of the delta between them.
  We evaluate three visual manipulations on mitigating sine illusions: dotted
lines, aligned gridlines, and offset gridlines via a user study.
  We asked participants to compare the deltas between two lines at two time
points and found aligned gridlines to be the most effective in mitigating sine
illusions.
  Using data from the user study, we produced a model that predicts the impact
of the sine illusion in line charts by accounting for the ratio of the vertical
distance between the two points of comparison.
  When the ratio is less than 50\%, participants begin to be influenced by the
sine illusion. This effect can be significantly exacerbated when the difference
between the two deltas falls under 30\%.
  We compared two explanations for the sine illusion based on our data: either
participants were mistakenly using the perpendicular distance between the two
lines to make their comparison (the perpendicular explanation), or they
incorrectly relied on the length of the line segment perpendicular to the angle
bisector of the bottom and top lines (the equal triangle explanation).
  We found the equal triangle explanation to be the more predictive model
explaining participant behaviors.
