title: Dimension containers when plotting Holoviews Curve and spread overlay in Holomap
body: <p>With the script below I expected to get a dropdown menu for changing the slope. i.e only showing one of the plots, not all three.</p>
  
  <pre><code>import numpy as np
  import holoviews as hv
  hv.extension('bokeh')
  
  slope = [1, 5, 10]
  
  def curve(slope):
      x = np.linspace(1,10)
      y = slope*0.5+x
      err = x*0.2
      return hv.Curve((x, y)) * hv.Spread((x,y,err))
  
  
  curve_dict = {r:curve(r) for r in slope}
  
  kdims = hv.Dimension(("slope", "slope"))
  
  hmap = hv.HoloMap(curve_dict, kdims=kdims).overlay()
  hmap
  </code></pre>
  
  <p>This return the following plot:</p>
  
  <p><a href="https://i.stack.imgur.com/uNUXB.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/uNUXB.png" alt="enter image description here"></a></p>
  
  <p>What am I missing here? I expected a dropdown menu, not all three graphs showing up.</p>