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[Bug]: Draggable legend with use_blit hides legend after legend picking #28129
Description
Bug summary
Using the legend picking example (https://matplotlib.org/stable/gallery/event_handling/legend_picking.html) and changing the next last line from leg.set_draggable(True) to leg.set_draggable(True, use_blit=True) then legend is not showing after picking it. Clicking again over its position makes it visible again.
Setting the legend draggable again (after being picked/clicked) with use_blit=False does not help.
Best regards, Daniel
Code for reproduction
# https://matplotlib.org/stable/gallery/event_handling/legend_picking.html
import matplotlib.pyplot as plt
import numpy as np
t = np.linspace(0, 1)
y1 = 2 * np.sin(2 * np.pi * t)
y2 = 4 * np.sin(2 * np.pi * 2 * t)
fig, ax = plt.subplots()
ax.set_title('Click on legend line to toggle line on/off')
(line1, ) = ax.plot(t, y1, lw=2, label='1 Hz')
(line2, ) = ax.plot(t, y2, lw=2, label='2 Hz')
leg = ax.legend(fancybox=True, shadow=True)
lines = [line1, line2]
map_legend_to_ax = {} # Will map legend lines to original lines.
pickradius = 5 # Points (Pt). How close the click needs to be to trigger an event.
for legend_line, ax_line in zip(leg.get_lines(), lines):
legend_line.set_picker(pickradius) # Enable picking on the legend line.
map_legend_to_ax[legend_line] = ax_line
def on_pick(event):
# On the pick event, find the original line corresponding to the legend
# proxy line, and toggle its visibility.
legend_line = event.artist
# Do nothing if the source of the event is not a legend line.
if legend_line not in map_legend_to_ax:
return
ax_line = map_legend_to_ax[legend_line]
visible = not ax_line.get_visible()
ax_line.set_visible(visible)
# Change the alpha on the line in the legend, so we can see what lines
# have been toggled.
legend_line.set_alpha(1.0 if visible else 0.2)
fig.canvas.draw()
fig.canvas.mpl_connect('pick_event', on_pick)
# Works even if the legend is draggable. This is independent from picking legend lines.
leg.set_draggable(True, use_blit=True)
plt.show()Actual outcome
The legend is not showing after picking it. Clicking again over its position makes it visible again.
Expected outcome
Legend is visible at all times.
Additional information
No response
Operating system
Windows
Matplotlib Version
3.8.1
Matplotlib Backend
TkAgg
Python version
3.9.7
Jupyter version
No response
Installation
pip
Activity
tacaswell commentedon May 22, 2024
The issue is that when you click on the line there are two pick callbacks that run: yours and the
on_pickfrom the DraggableOffsetbox. They both have afig.canvas.draw()and the one in offsetbox sets the legend to be animated (which excludes it from the normal draw process so we can grab the background to re-draw on top of for blitting to work). Thus the sequence of events seems to be:on_releasecallback in offset box makes the legend not-animated againYou can break the cycle by adding
just above
fig.canvas.draw()in your callback (and it is better to usefig.canvas.draw_idle()in your case). It will still do weird things if you click on the line and then drag (as the cached background the dragging code has includes your line being visible), but that is better than the legend disappearing!This is an unfortunate thing about the (low) level of abstraction we provide on top of blitting. Because it is all managed with local state it is really easy to have two things step on each others toes like this.
[-]Draggable legend with use_blit hides legend after legend picking [Bug]: [/-][+][Bug]: Draggable legend with use_blit hides legend after legend picking[/+]QuLogic commentedon Jun 5, 2026
This should have been fixed by #28271.