Matplotlib drag overlapping points interactively

Question

In my case, I only want to drag one point each time. However, since the two points are heavily overlapping, dragging one point would cause another point to be dragged. How c

Answer 1

I would synchronize which artist is currently active through a single class that works with multiple artists.

It's easiest to use the pick_event for this. It also makes it easier to generalize to other artists. As an example:

import matplotlib.pyplot as plt
import matplotlib.patches as patches

class DraggablePoints(object):
    def __init__(self, artists, tolerance=5):
        for artist in artists:
            artist.set_picker(tolerance)
        self.artists = artists
        self.currently_dragging = False
        self.current_artist = None
        self.offset = (0, 0)

        for canvas in set(artist.figure.canvas for artist in self.artists):
            canvas.mpl_connect('button_press_event', self.on_press)
            canvas.mpl_connect('button_release_event', self.on_release)
            canvas.mpl_connect('pick_event', self.on_pick)
            canvas.mpl_connect('motion_notify_event', self.on_motion)

    def on_press(self, event):
        self.currently_dragging = True

    def on_release(self, event):
        self.currently_dragging = False
        self.current_artist = None

    def on_pick(self, event):
        if self.current_artist is None:
            self.current_artist = event.artist
            x0, y0 = event.artist.center
            x1, y1 = event.mouseevent.xdata, event.mouseevent.ydata
            self.offset = (x0 - x1), (y0 - y1)

    def on_motion(self, event):
        if not self.currently_dragging:
            return
        if self.current_artist is None:
            return
        dx, dy = self.offset
        self.current_artist.center = event.xdata + dx, event.ydata + dy
        self.current_artist.figure.canvas.draw()

if __name__ == '__main__':
    fig, ax = plt.subplots()
    ax.set(xlim=[-1, 2], ylim=[-1, 2])

    circles = [patches.Circle((0.32, 0.3), 0.2, fc='r', alpha=0.5),
               patches.Circle((0.3, 0.3), 0.2, fc='b', alpha=0.5)]
    for circ in circles:
        ax.add_patch(circ)

    dr = DraggablePoints(circles)
    plt.show()