histogram.value()

histogram.value([value])

If value is specified, sets the value accessor to the specified function or constant and returns this histogram generator. If value is not specified, returns the current value accessor, which defaults to the identity function.

When a histogram is generated, the value accessor will be invoked for each element in the input data array, being passed the element d, the index i, and the array data as three arguments. The default value accessor assumes that the input data are orderable (comparable), such as numbers or dates. If your data are not, then you should specify an accessor that returns the corresponding orderable value for a given datum.

This is similar to mapping your data to values before invoking the histogram generator, but has the benefit that the input data remains associated with the returned bins, thereby making it easier to access other fields of the data.