stack.keys()

stack.keys([keys])

If keys is specified, sets the keys accessor to the specified function or array and returns this stack generator. If keys is not specified, returns the current keys accessor, which defaults to the empty array. A series (layer) is generated for each key. Keys are typically strings, but they may be arbitrary values. The series’ key is passed to the value accessor, along with each data point, to compute the point’s value.