Consider the chart below:
When writing percent in the title, and % on the axis marks, is this redundant? Is the word amount preferable?
I haven’t seen this addressed in any style guides such as the APA guidelines.
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Percentage (parts per hundred) and, more broadly, abbreviations like ppm (parts per million) or ppb (parts per billion) do not describe any quantity, they are just multipliers for dimensionless quantities.
In a graph, instead, you should accurately describe the quantity being represented. In the example given in your question, a more appropriate title would probably be: "Fraction of the widgets that are blue".
In the SI brochure, §5.3.7, the document that defines the International System of Units, it is stated that
In mathematical expressions, the internationally recognized symbol % (percent) may be used with the SI to represent the number 0.01. Thus, it can be used to express the values of dimensionless quantities.
Phrases such as "percentage by mass", "percentage by volume", or "percentage by amount of substance" should not be used; the extra information on the quantity should instead be conveyed in the name and symbol for the quantity.
The percent symbol can, instead, appear in the tick labels.