While legibility is about readability as a result of how something is written/appears, readability is also affected by length. If a chart is bigger and more complicated it can convey more nuance—at the cost of being harder to read, and taking longer. While font can usually be changed without disrupting the message, it’s more work to do this for length, if it can be done at all without trading off in way that aren’t improvements. (If I invented a language for conveying messages more succinctly in visual form, and this message as a whole was 13876, that wouldn’t decrease the amount of time it takes to read (and understand it) it.)
While legibility is about readability as a result of how something is written/appears, readability is also affected by length. If a chart is bigger and more complicated it can convey more nuance—at the cost of being harder to read, and taking longer. While font can usually be changed without disrupting the message, it’s more work to do this for length, if it can be done at all without trading off in way that aren’t improvements. (If I invented a language for conveying messages more succinctly in visual form, and this message as a whole was 13876, that wouldn’t decrease the amount of time it takes to read (and understand it) it.)