Add link_selections function for automated linked selections

[jonmmease]

Overview

This PR adds a holoviews.selection.link_selections function that is an initial implementation of the automatic linked selection approach proposed in #3842.

This function can be used to transform HoloViews objects into new objects with linked selections enabled.

Dependencies

This PR depends on the following open PRs, and will be rebased once these are merged.

• Support selections across all dimensions in element.dataset #3924
• Add SelectionExpr stream #3927

Example notebook

For examples, see the notebook at https://anaconda.org/jonmmease/link_selections_pr/notebook.

Please play around with this notebook to get a feel for the workflow and capabilities and report back on how things look @jbednar @philippjfr @jlstevens.

Highlights

Linked Points plots with Bokeh backend:

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Linked Points, Histogram, Scatter3d, and Table layout with Plotly backend:

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Linked Violin, Points and Bivariate overlay, and Distribution layout with Bokeh backend:

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Selections on datashaded elements

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Linked plots across notebook output cells, and updating selection color after display:

Extensions

There is a lot more functionality that can be built on this foundation. Here are some directions:

• The link_selections function only stores the latest selection. But the _base_link_selections base class is more general, and other subclasses can be added for cross filtering, and glue-style multi-color brushing.
  • For cross-filtering, I really like the 4-color approach used in https://plotly.github.io/plotly.js-crossfilter.js/. I personally find this easier to make sense of compared to the 2-color approach used in http://crossfilter.github.io/crossfilter/ where the "filtered" data is fully removed from the plot.
  • For multi-colored brushing, the inspiration is the glue project http://docs.glueviz.org/en/stable/getting_started/index.html?highlight=subsets#defining-subsets. This would require a small Panel GUI interface for choosing the current subset color and the subset mode (union, intersection, or subtraction).
• Add Bounds element to display the selected region after a selection is completed. I think this will largely be a requirement for the cross-filter mode, otherwise it would be hard to make sense of the union of the selected regions across plots.
• Selections could be displayed on grid elements (e.g. vector field, raster) if we had an element.where operation (Add element.where method support dim expressions #3933).
• For full hvplot compatibility, will need to review hvplot to make sure source dataset information flows through all of the produced elements. This happens for some, but not all, elements at the moment.
• Investigate geoviews compatibility
• Dask optimization of dim expressions and histogram operation to make selection with datashader + histogram elements with Dask DataFrames scale well.
• Support passing full Panel layouts to link_selections and traverse them to find HoloViews panes. This way a Panel layout with HoloViews panes that use different backends could be linked together.
• Add a new BoundsPoly stream that is analogous to BoundsXY but for a polygon rather than a box. Use this to support defining selection regions using lasso selection.

Let me know which of these you think are capabilities that we really should have for the initial release. Thanks!

[jbednar]

Fabulous! Can we have all of those things? :-) It is hard to choose!

[jonmmease]

after rebasing on the pipeline support, it's also possible to automate linked selections through custom aggregation steps. e.g.

bars = iris_ds.reindex(
    kdims='species_id', vdims=['sepal_length']
).to(hv.Bars).aggregate(function=np.sum)
link_selections(bars + points2)

[philippjfr]

after rebasing on the pipeline support, it's also possible to automate linked selections through custom aggregation steps

Very, very neat.

[jbednar]

Cool!!

[johnzzzzzzz]

nice

[jonmmease]

All of the dependent PRs have been merged, so I squashed and rebased this PR on master. Review/feedback appreciated!

[philippjfr]

Started reviewing but it looks to me like something went wrong during a merge or rebase and a number of preceding PRs are being reverted by this one.

[philippjfr]

Once that's fixed I'm fine with merging this. I'd like to see a user guide on using linked selections along with a few reference and gallery examples but that can happen in later PRs.

[jonmmease]

Thanks for catching that @philippjfr, I think it's straightened out now and I won't force push on this PR again in case you want to leave comments inline. Thanks for taking a look!

[jbednar]

Oh, that explains the seemingly unrelated changes! The diffs make much more sense now!

[philippjfr]

There appears to be some Python 2 issue.

[jonmmease]

Yeah, those errors are a bit confusing. I'll build a local Python 2 environment and see what I can figure out.

[philippjfr]

Very excited about this! Going to merge now.

[jonmmease]

Great! Thanks for tracking down the python 2 issue!