Memory leak

Describe the issue

Plotting a 30M row data frame causes memory only to increase. Also memory isn't collected by gc.collect.
As discussed with matplotlib guys in https://github.com/matplotlib/matplotlib/issues/21259 (and Jupyter guys: https://github.com/jupyterlab/jupyterlab/issues/11182) - we tested and it works as intended. I tested further and think I found the issue - %matplotlib widget:

%matplotlib widget  # if this is commented out, gc.collect releases the memory

import gc

import matplotlib.pyplot as plt
import numpy as np
import pandas as pd


zeros = pd.Series(np.zeros(30_000_000))
df = pd.concat([zeros, zeros, zeros, zeros], axis=1)


FROM = 0
TO = 10_000_000  # df.shape[0]
color_spread = "grey"

fig, axs = plt.subplots(1, 1, sharex=True)

ax2 = axs.twinx()
ax2.plot(df[df.columns[0]][FROM:TO], color="blue")
ax2.plot(df[df.columns[1]][FROM:TO], color="red")
ax2.tick_params(axis="y", labelcolor="red")

ax1 = axs.twinx()
ax1.plot(df[df.columns[2]][FROM:TO], color="orange")
ax1.plot(df[df.columns[3]][FROM:TO], color="yellow")

fig.tight_layout()
plt.show()
gc.collect()

Versions

3.7.10 | packaged by conda-forge | (default, Feb 19 2021, 16:07:37) 
[GCC 9.3.0]
ipympl version: 0.7.0
jupyter core     : 4.7.1
jupyter-notebook : 6.4.3
qtconsole        : not installed
ipython          : 7.27.0
ipykernel        : 6.4.1
jupyter client   : 7.0.2
jupyter lab      : 3.1.11
nbconvert        : 6.1.0
ipywidgets       : 7.6.5
nbformat         : 5.1.3
traitlets        : 5.1.0
Config option `kernel_spec_manager_class` not recognized by `ListNBExtensionsApp`.
Known nbextensions:
  config dir: /home/test/PROGS/miniconda3/envs/puma-lab/etc/jupyter/nbconfig
    notebook section
      jupyter-matplotlib/extension  enabled 
      - Validating: OK
      nbdime/index  enabled 
      - Validating: OK
      jupyter-js-widgets/extension  enabled 
      - Validating: OK
      nbextensions_configurator/config_menu/main  enabled 
      - Validating: problems found:
        - require?  X nbextensions_configurator/config_menu/main
      contrib_nbextensions_help_item/main  enabled 
      - Validating: OK
    tree section
      nbextensions_configurator/tree_tab/main  enabled 
      - Validating: problems found:
        - require?  X nbextensions_configurator/tree_tab/main
Config option `kernel_spec_manager_class` not recognized by `ListLabExtensionsApp`.
[W 2021-10-07 12:20:00.836 LabApp] Config option `kernel_spec_manager_class` not recognized by `LabApp`.
JupyterLab v3.1.11
/home/test/PROGS/miniconda3/envs/puma-lab/share/jupyter/labextensions
        jupyter-matplotlib v0.9.0 enabled OK
        nbdime-jupyterlab v2.1.0 enabled OK
        @jupyterlab/git v0.32.2 enabled OK (python, jupyterlab-git)
        @pyviz/jupyterlab_pyviz v2.1.0 enabled OK (python, pyviz_comms)
        @jupyter-widgets/jupyterlab-manager v3.0.1 enabled OK (python, jupyterlab_widgets)

Other labextensions (built into JupyterLab)
   app dir: /home/test/PROGS/miniconda3/envs/puma-lab/share/jupyter/lab

提供された最小再現例を Jupyter notebook で開始し、%matplotlib widget を有効にした場合とコメントアウトした場合のメモリ動作を比較します。ipympl ウィジェット統合を調査し、プロット後に gc.collect() を呼び出すと、データフレームとプロットのメモリが期待どおり解放されることを確認します。