Add fancy_layout option to HoloViews pane

[philippjfr]

The current HoloViews pane generates a simple layout of the plot and the set of widgets generated from that plot. This makes it easy to compose these two components in different ways but it doesn't match the look that the old-style HoloViews widgets use. Since we want to swap out the widgets in HoloViews with as little disruption as possible this PR adds a fancy_layout option which tries to match the look and feel of the original widget implementation.

As a simple example here is the output of a HoloMap with this option enabled:

[jbednar]

I se this is merged already, but I'm wondering whether fancy is the right term for this arrangement. Fancy makes it sound like it's better than the default, but really it's just different, as far as I can see. Could we use a more neutral word here, like shaded_widgets?

[philippjfr]

Happy to change it, about to open another PR to improve the scrubber layout anyway, but -1 on shaded. Open to other suggestions.

[philippjfr]

The differences are:

1. The widgets are inside a widget box (which is shaded and gray)
2. The plot and widgets are horizontally centered and the margins are responsive
3. The widget box is vertically centered

[philippjfr]

Also for context, the "fancy" bit is borrowed from matplotlib's fancybox option on legends: https://matplotlib.org/gallery/recipes/transparent_legends.html

[jbednar]

I don't think it shares much in common with the mpl fancybox option, which isn't grey or offset like that. Maybe center_shaded? I'm just looking for something more literal and with no valence. But it's only a slight preference, so if there's no literal name you are happy with, then I guess just keep it fancy.

[philippjfr]

Cc: @jsignell @jlstevens @jonmmease Any opinions or suggestions?

[jsignell]

What about original_layout or classic_layout

[philippjfr]

I like classic_layout.

[jbednar]

classic_layout is fine if it's not something we plan to support forever or expect new users to consider using.

[philippjfr]

classic_layout is fine if it's not something we plan to support forever or expect new users to consider using.

We do plan to support it forever, it's the default layout HoloViews will use to render its widgets. Panel users should generally use the default layout so they can compose it themselves more easily.

[jonmmease]

I don't have strong feeling on the name itself, but should we consider this to be an enumeration rather than a boolean? Something like layout_style='holoviews', and then leave open the possibility of other styles in the future.

[philippjfr]

Something like layout_style='holoviews', and then leave open the possibility of other styles in the future.

Not a bad idea either.

[philippjfr]

Currently the options would be 'simple' and 'classic' or 'holoviews'.

[jonmmease]

As a disclaimer, I haven't looked at it closely enough to have an opinion on whether we would want to have more options like this in the future. I just know I've often come to regret boolean arguments 🙂

[philippjfr]

As a disclaimer, I haven't looked at it closely enough to have an opinion on whether we would want to have more options like this in the future

I would certainly have to refactor the code and come up with some better representation to express custom layouts because the code is already getting unwieldy with two options but I think more flexibility would be a good thing. People certainly have asked for more flexibility in HoloViews and while they can already achieve that if they use Panel directly and lay things out themselves I don't think it would hurt to include a few more canned layouts they can pick from.

[philippjfr]

If no one objects I'm going with:

layout_style = param.ObjectSelector(default='simple', objects=['simple', 'classic'], doc="""
    The default layout of the plot and widgets. The default is a 'simple' layout to make
    it easy to compose the plot and the widgets manually.""")

[jlstevens]

The last suggestion above looks good to me in terms of a layout_style parameter. I think 'simple' is clear enough as a name (the current panel behavior afaik) but 'classic' isn't too informative as to what that really means.

I am not a fan of 'holoviews' as a value and 'adjoint' is probably confusing given holoviews uses that word as well. I'm wouldn't let 'classic' block this PR but I would like to try and think of something else if possible (tricky!).

[philippjfr]

Could call it centered I guess but there could be a number of centered arrangements. One problem is that it also interacts with the widget_type parameter. Maybe we could add center_right, center_left, center_bottom, center_top, left, right, bottom and top options instead. HoloViews would use center_right for regular widget and center_bottom for the scrubber.

[jlstevens]

Those options make sense to me and generalize panel nicely. Of course it is also more work to do but this would be my preferred approach in terms of offering a general and intuitive API...

[jbednar]

I was reading the recent discussion starting from the top and independently thought of center_right before reading @philippjfr's comment, so I vote for that -- that way people can declare an appropriate default mapping to a layout that they prefer in general, and of course then they can always repack things manually using Panel if they wish.