title: matplotlib customizing axes shadow
body: <p>I would like to customize the axes shadow in a matplotlib plot. the shadow looks unpropotionate to the other patches i am adding to the plot. I need dx and dy to be at least 4 to show the perspective I am looking for. as the only parameter is a boolean axes there is no way to control the shadow depth. I am trying to create the Shadow object myself and adding it to the legend but its not working correctly.
  please find herein an exemple</p>
  
  <pre><code>import wx
  import numpy as np
  from matplotlib.backends.backend_wxagg import FigureCanvasWxAgg
  from matplotlib.figure import Figure
  from matplotlib.legend import Legend
  from matplotlib.patches import Shadow
  from matplotlib.axes import Axes
  from matplotlib.lines import Line2D
  
  class Frame(wx.Frame):
      def __init__(self):
          # initialize
          wx.Frame.__init__(self, None)
          # create main sizer
          mainSizer = wx.BoxSizer(wx.VERTICAL)
          # create figure, canvas and matplotlib toolbar
          self.figure = Figure(figsize=(4.5,4), dpi=None)
          self.canvas = FigureCanvasWxAgg( self, -1, self.figure )
          # add canvas to sizer
          mainSizer.Add(self.canvas, proportion=1, flag=wx.ALL|wx.EXPAND)
          self.SetSizer(mainSizer)
          mainSizer.Fit(self)
          self.Show(True)  
  
  # create parabola data to plot
  data = np.array(range(100))**2/100.
  # start the plot
  app = wx.App(0)      
  frame = Frame()
  axes = frame.figure.add_axes((0.1,0.1,0.8,0.8))
  # add line 
  line = Line2D(xdata=range(len(data)), ydata=data)
  axes.add_line(line)
  # set axes limits
  axes.set_xlim(left=0,right=len(data))
  axes.set_ylim(bottom=data[0],top=data[-1])
  # create legend
  l = Legend(axes,[line],['parabola'], shadow=False)
  s = Shadow(l.legendPatch, 5, -5)
  # add legend and shadow to axes
  axes.legend_ =l
  axes.add_patch(s)
  # draw canvas                 
  frame.canvas.draw()
  
  app.MainLoop()
  </code></pre>
  
  <p>when the frame shows up on the screen the shadow is not drawn, It appear when I start resizing and sets behind the legend or near is in a weird way depending on how you resize.
  this is a snapshot of my weird shadow before and after resizing</p>
  
  <p><img src="https://i.stack.imgur.com/nwF3I.png" alt="This is the plot just when it got drawn before any resizing. no shadow!!!"></p>
  
  <p><img src="https://i.stack.imgur.com/zS3Jd.png" alt="This is the plot after some resizing. Shadow is disproportionate and resizes weirdly"></p>