Memory leak

Nobody has claimed this yet.

## 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`:
```python
%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

```

Start with the supplied minimal reproducer in a Jupyter notebook, comparing memory behavior with `%matplotlib widget` enabled and commented out. Investigate the ipympl widget integration and verify that after plotting and calling `gc.collect()`, the data frame and plot memory are released as expected.

Written by the indexing model from the issue text.