The Oblique Effect: Humans are inconsistent at discerning the angle of lines.
They are better at measuring angles closer to horizontal or the vertical. The below plot shows error changing with line section angle.
As a best practice, quantize angles in designs to 30 degree increments and start with vertical and horizonal reference points.
Although humans have trouble measuring absolute angles, they can easily discern between two reference angles. This is known as the pop out effect. This can be used to discover or highlight differences in small multiples.
Universal Principles of Design; Lidwell, Holden, Butler
The guidelines do not provide a good sense of how to use multiple lines to reveal patterns. In these cases, using the aggregate effect of many, subtle angles can be quite effective and would be ruined by maintaining large differences between angles.
In the above plot of weather data, being able to discern discreet angles is not important. Rather, the subtle variations allow the plot to communicate a high volume of information about weather patterns.