title: Why aren't my quiver arrows pointing the correct direction?
body: <p>I am struggling to understand why my quiver arrows aren't pointing in the proper direction when plotting wind direction. I have provided an MWE below that has arrays dates, wind_dir, and wind_speed and plots their values over time. I use angles parameter as 'xy' so that arrows point from (x,y) to (x+u,y+v). Anyone have a minute to help?</p>
  
  <pre><code>from matplotlib.dates import date2num
  import numpy as np
  import matplotlib.pyplot as plt
  from datetime import datetime
  from matplotlib.dates import DateFormatter, YearLocator, MonthLocator, DayLocator, HourLo
  #from file.py import *
  
  dates = [datetime(2020, 4, 22, 0, 53),
  datetime(2020, 4, 22, 1, 53),
  datetime(2020, 4, 22, 2, 53),
  datetime(2020, 4, 22, 3, 53)]
  
  idx = date2num(dates)
  
  wind_dir = [170., 150., 130., 230.]
  
  wind_speed = [3.6,  2.57, 4.12, 7.09 ]
  
  U = wind_speed*np.cos(wind_dir)
  V = wind_speed*np.sin(wind_dir)
  
  fig, ax1 = plt.subplots()
  
  qq = ax1.quiver(idx, np.ones(len(idx))*1,U,V,wind_speed,
     pivot='mid',angles='xy',cmap=plt.cm.jet)
  
  plt.colorbar(qq, cmap=plt.cm.jet)
  ax1.plot(dates, wind_speed)
  
  # format the x ticks
  months = MonthLocator()
  days = DayLocator()
  hours = HourLocator(byhour=[0,1,2,3])
  hours_each = HourLocator() # Find all hours
  dateFmt = DateFormatter('%b %d\n%H:%M') # Tick label format style
  ax1.xaxis.set_major_locator(hours)
  ax1.xaxis.set_major_formatter(dateFmt) # Set the x-axis labels
  ax1.xaxis.set_minor_locator(hours_each)
  plt.xticks(rotation=30)
  plt.show()
  </code></pre>