mbostock
commented
7 years ago The value accessor returns the value for the corresponding datum and key, so I think it’s named accurately. The + is there just to coerce the value to a number in case the value accessor doesn’t return a number; a well-behaved value accessor should return a number, so in the normal case the + does nothing.
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This is just a question. Value() method does not play a role of "value" but just an accessor. Transform to numerical value is done by "+" inside main function. Isn't it a little confusing? I know good boys must preprocess data before feeding stack().