Introduced on day 8
Principle of proportional ink
The size of shaded areas in a chart should be proportional to the values they represent, which is why bar and area charts must start at zero.
In detail
Bars encode by length, so a truncated axis exaggerates differences; dots and lines encode by position and can be zoomed if the zoom is visible. The term is credited to Bergstrom and West (2016) in Wilke, Fundamentals of Data Visualization, ch. 17.