The clean function is not properly removing the values.

if a var (wind speed) is nan or equal to 0, this data is removed from var array but also from dir array

But...

Example:

>>> direction = np.array([15.0])
>>> var = np.array([np.nan])
>>> windrose.clean(direction, var)
(array([15.]), array([nan]))

Data is not removed in case that a var is nan.

It should return

(array([], dtype=float64), array([], dtype=float64))