How to set quiverkey length scaling with xy data in matplotlib?

Question

I am trying to set the absolute quiverkey arrow-length matching a specific arrow-length of my quiver data. Can somebody enlighten me how the quiverkey argument U works or at least how to scale it to my needs?

Background

Python version: 3.6.5

matplotlib version: 3.0.0

Minimal example

I've slightly modified the example from: https://matplotlib.org/gallery/images_contours_and_fields/quiver_simple_demo.html#sphx-glr-gallery-images-contours-and-fields-quiver-simple-demo-py

import matplotlib.pyplot as plt
import numpy as np

X = np.arange(-10, 11, 2)
Y = np.arange(-10, 11, 2)
U, V = np.meshgrid(X, Y)

# calculate length of each arrow
data_arrow_length = np.abs(np.sqrt(np.square(U[:]) + np.square(U[:])))

# set displayed arrow length for longest arrow
displayed_arrow_length = 2

# calculate scale factor for quiver
scale_factor = np.max(data_arrow_length)/displayed_arrow_length

# mysterious arrow length for quiverkey
quiverkey_length = 200

fig, ax = plt.subplots()
q = ax.quiver(X, Y, U, V, pivot='middle',
              units='xy', width=0.1,
              scale=scale_factor, scale_units='xy')
ax.quiverkey(q, X=0.3, Y=1.1, U=quiverkey_length,
             label='Quiver key, length = 10', labelpos='E')

plt.show()

This way I can control the maximal arrow-length displayed with quiver using displayed_arrow_length. In this example the maximal data_arrow_length is approx. 14 and is displayed with an arrow length of 2 based on xy-data.

Problem

If I run the above example I get:

However, if I maximize the figure window I get:

The quiverkeyarrow-length does not scale with my quiver arrows and I had to use an randomly high number for quiverkey_length to appear altogether.

Question

I would like to set the quiverkey arrow length to a specific value as displayed with quiver, for example 10. However, it does not appear to scale with the data at all. How can I achieve this?

Answer 1

It appears this was a bug in matplotlib which has been fixed now.

related github issue