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matplotlib.axes.Axes.start_pan
Axes.start_pan(x, y, button)- 
    
Called when a pan operation has started.
x, y are the mouse coordinates in display coords. button is the mouse button number:
- 1: LEFT
 - 2: MIDDLE
 - 3: RIGHT
 
Note
Intended to be overridden by new projection types.
 
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 http://matplotlib.org/2.0.2/api/_as_gen/matplotlib.axes.Axes.start_pan.html