On datetime series, this allows for setting the pointInterval to irregular time units, day, month and year. A day is usually the same as 24 hours, but pointIntervalUnit also takes the DST crossover into consideration when dealing with local time. Combine this option with pointInterval to draw weeks, quarters, 6 months, 10 years etc.

Please note that this options applies to the series data, not the interval of the axis ticks, which is independent.