axis.tickSizeOuter()

axis.tickSizeOuter([size])

If size is specified, sets the outer tick size to the specified value and returns the axis. If size is not specified, returns the current outer tick size, which defaults to 6. The outer tick size controls the length of the square ends of the domain path, offset from the native position of the axis. Thus, the “outer ticks” are not actually ticks but part of the domain path, and their position is determined by the associated scale’s domain extent. Thus, outer ticks may overlap with the first or last inner tick. An outer tick size of 0 suppresses the square ends of the domain path, instead producing a straight line.