First and last thresholds are set to data extent (not explicitly stated limits)

[mkfreeman]

Based on the documentation, Thresholds are defined as an array of values [x0, x1, …]. Given that, I would expect the bin limits (x0, x1) to match the threshold arguments. However, the x0 of the first bin is set to the data minimum, and the x1 of the last bin is set to the data maximum. For example:

// expect x0:1, x1: 2, but returns x0: 1.5, x1: 1.5
d3.bin().thresholds([1, 2])([1.5]).map(d => ({x0: d.x0, x1: d.x1}))

[mbostock]

See bin.domain:

Any threshold values outside the domain are ignored. The first bin.x0 is always equal to the minimum domain value, and the last bin.x1 is always equal to the maximum domain value.

[mkfreeman]

Thanks for clarifying!