subcoordinate_y: respect ylim

[maximlt]

Fixes #5952

I want to wait for #6122 to be merged to fully set this PR. At the moment I think you can't zoom-out from the ylim which isn't good.

[philippjfr]

Looks good.

[maximlt]

Hmm so an issue at the moment is that when ylim is set on the Overlay the y-range cannot be changed by the user as the zoom tool is associated with the subcoordinate_y elements at a level of 1. I need to think more about this issue.

[maximlt]

ylim (xlim too) isn't a hard-bound in HoloViews unless apply_hard_bounds is set to True (#6056), instead, it sets the initial y-bounds. So users can freely zoom and pan outside of the initial view when ylim is set.

Let's dive more into the zoom UX with subcoord-y overlays!

By default, the user gets global Box Zoom and Pan zoom tools controlling the whole viewport (level 0) and a Wheel Zoom controlling the subcoord-y subplots (y-axis at level 1) => Users can zoom in globally with the Box Zoom but can't zoom out at level 0 except with the Reset tool. This is a problem that was already there and that is made more obvious by this PR.

import holoviews as hv
import numpy as np
hv.extension('bokeh')

x = np.linspace(0, 10*np.pi)
curves = [
    hv.Curve((x + i*np.pi/2, np.sin(x)), label=f'Line {i}').opts(
        subcoordinate_y=True,
    )
    for i in range(3)
]
hv.Overlay(curves).opts(show_legend=False)

Users can add Zoom In and Zoom Out tools that control the subcoord-y subplots (y-axis at level 1).

curves = [
    hv.Curve((x + i*np.pi/2, np.sin(x)), label=f'Line {i}').opts(
        subcoordinate_y=True, tools=['zoom_in', 'zoom_out'],
    )
    for i in range(3)
]
hv.Overlay(curves).opts(show_legend=False)

A little detour on a normal overlay behavior: it is not possible to directly add global tools to an overlay. This seems to be unrelated to subcoord-y.

overlay = hv.Curve([-1, -2, -3]) * hv.Curve([1, 2, 3])
overlay.opts(tools=['zoom_in'])
# No zoom_in tool!

For normal overlays, the usual workaround, I think, is to directly add a tool to a least one element of the overlay.

overlay = hv.Curve([-1, -2, -3]) * hv.Curve([1, 2, 3]).opts(tools=['zoom_in'])
overlay

This workaround doesn't work with subcoord-y overlays, it will not add a global Zoom In tool, instead, it'll just allow to zoom in on one subplot. That's because wheel_zoom, zoom_in and zoom_out tools are special-cased in the scope of a subcoord-y overlay, they're turned into subcoord-y zoom tools (y-axis at level 1). In this case, the workaround is to add a custom zoom instance to at least one element of the overlay.

from bokeh.models import WheelZoomTool

curves = [
    hv.Curve((x + i*np.pi/2, np.sin(x)), label=f'Line {i}').opts(
        subcoordinate_y=True, tools=[WheelZoomTool()],
    )
    for i in range(3)
]
hv.Overlay(curves).opts(show_legend=False, ylim=(0, 1))

With grouped subcoord-y overlays, the same issues and workaround apply. Except that, at least at the moment, there are more zoom tools in the toolbar.

curves = [
    hv.Curve((x + i*np.pi/2, np.sin(x)), group=group, label=f'{group} Line {i}').opts(
        subcoordinate_y=True, tools=[WheelZoomTool()],
    )
    for group in 'ABC'
    for i in range(3)
]
hv.Overlay(curves).opts(show_legend=False, ylim=(0, 1))

In conclusion:

1. This PR exposes more the fact that setting a tool directly on an overlay is a no-op. I don't know if it's by design, a bug, or that it was just never implemented.
2. The workaround for subcoord-y overlays is to pass a custom wheel zoom / zoom in / zoom out instance.
3. This PR also exposes more of the issue users will have with differentiating a normal zoom tool with a subcoord-y zoom tool (y-axis at level 1). With @hoxbro we discussed creating a custom icon, derived from the existing icons.

My take on these:

1. Looks like this deserves to be its own issue (I haven't found one)
2. I'd be okay documenting this workaround, as it's similar to the usual workaround with setting a tool on an overlay, at least until 1. is resolved.
3. I also think it deserves its own issue, the "hard" work being to create custom wheel zoom / zoom in / zoom out icons (no designer here 🙃 )

cc @droumis some food for thought

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.55%. Comparing base (c3c211e) to head (1f7d510).
Report is 34 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6190      +/-   ##
==========================================
- Coverage   88.70%   87.55%   -1.15%     
==========================================
  Files         314      319       +5     
  Lines       65993    67167    +1174     
==========================================
+ Hits        58537    58808     +271     
- Misses       7456     8359     +903     

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