d3.tickStep()

d3.tickStep(start, stop, count)

Returns the difference between adjacent tick values if the same arguments were passed to ticks: a nicely-rounded value that is a power of ten multiplied by 1, 2 or 5. Note that due to the limited precision of IEEE 754 floating point, the returned value may not be exact decimals; use d3-format to format numbers for human consumption.