Finalize visualizations to avoid memory leaks

[domoritz]

Vega-Embed and Vega may be holding on to memory after a visualization is no longer needed. To fix this, you can call finalize on a Vega-Embed result (since 6.2) or Vega view.

When you know that a vis is no longer needed (i.e. the react component is unmounted), we should call finalize.

[jakevdp]

Sounds good – current release targets vega-embed 5, so we'll fix this in altair 4.0.

[jakevdp]

How would one know that "the visualization is no longer needed"?

[domoritz]

Good question. Jupyterlab finalizes a view (I am changing this to be the embed result) when a cell is overwritten (https://github.com/jupyterlab/jupyterlab/blob/master/packages/vega5-extension/src/index.ts#L87) or when the cell is disposed of (https://github.com/jupyterlab/jupyterlab/blob/master/packages/vega5-extension/src/index.ts#L124).