matplotlib.axes.Axes.start_pan
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Axes.start_pan(self, x, y, button)
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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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Licensed under the Matplotlib License Agreement.
    https://matplotlib.org/3.2.2/api/_as_gen/matplotlib.axes.Axes.start_pan.html