Term used to describe two lines appearing closer to each other when lines are at a greater gradient Forgive me if this is outside of the scope of Cross Validated, but it seemed relevant on the visualization front.  As described here (page 3) there exists a phenomenon when gauging the distance between two lines on a plot to compare the shortest distance between the two lines, rather than the vertical distance.  This leads to underestimation of the distance between lines at certain gradients.  To illustrate, the following graphic is from the aforementioned link:

The difference on the y-axis between these two lines is uniform across all x values, and yet the difference appears to shrink.  I am curious as to whether this phenomenon has an official name and if so, whether there is any kind of canonical reference paper/citation describing the phenomenon in more detail?
 A: This psycho-visual problem is the consequence of the mind making effective bisquare approximations when thinking about "distance".  It is most often addressed by plotting the variable of interest, in this case the difference, as its own variable.  Personally I would put this into a subplot with y-axis label as "difference between domestic and international" or such.
A: It would be interesting to see what the Cognitive Sciences stack exchange calls it, and I am not an expert in this, but I have been reading on it recently, and I think you would be justified in calling it a "subjective contour illusion".
The textbook Vision Science focuses on illusory contours like Kanizsa's Triangle, where viewers perceive a white triangle in an image that shows only lines and pac men:

It seems to me that mentally filling in the area between the two curves is just what is going wrong when viewers underestimate the gap between the lines in your example.  If it doesn't have a better name already, let's call it a subjective contour illusion.
