Unexpected behaviour when using alt.Scale(domain=selection) with both x and y axes

[jonathangjertsen]

I want to create a chart with a "minimap" similar to https://altair-viz.github.io/gallery/histogram_responsive.html, but where the selection is a box that I can click and drag (i.e., encodings=["x", "y"] when calling selection_interval).

The zooming works along the x-axis if I recreate the example, setting scale=alt.Scale(domain=zoom) for the x-axis only:

import numpy as np, pandas as pd, altair as alt

t = np.linspace(0, 100, 1000)
data = pd.DataFrame({ "t": x, "V": np.sin(x) })

zoom = alt.selection_interval(encodings=["x", "y"])

minimap = alt.Chart(data).mark_line().add_selection(zoom).encode(x="t", y="V").properties(height=100)
detail = alt.Chart(data).mark_point().encode(
    x=alt.X("t", scale=alt.Scale(domain=zoom)),
    y="V"
)

minimap & detail

but if I change y="V" to y=alt.Y("V", scale=alt.Scale(domain=zoom)), the limits on the Y axis are set to the x-axis bounds of the selection rather than the y-axis bounds.

After looking at https://vega.github.io/vega-lite/docs/selection.html#scale-domains I found a workaround that does exactly what I want:

detail = alt.Chart(data).mark_point().encode(x="t", y="V")
detail.encoding.x.scale = {"domain": {"selection": zoom.name, "encoding": "x"}}
detail.encoding.y.scale = {"domain": {"selection": zoom.name, "encoding": "y"}}

Is there a better way to do it though?

[jakevdp]

There's not a great higher-level syntax for this yet, but you can accomplish it using:

detail = alt.Chart(data).mark_point().encode(
    x=alt.X("t", scale=alt.Scale(domain={'selection': zoom.name, 'encoding': 'x'})),
    y=alt.Y("V", scale=alt.Scale(domain={'selection': zoom.name, 'encoding': 'y'}))
)

[jakevdp]

Going forward, we could probably make domain=zoom.x and domain=zoom.y shorthands for this, but we'd have to think about how this interacts with other places selection objects are used.

[jonathangjertsen]

Thanks!

Would it make sense to automatically set 'encoding': 'y' as the default in usages like alt.Y(..., scale=alt.Scale(domain=selection))? And 'encoding': 'color' when used in alt.Color(..., scale=alt.Scale(domain=selection)), etc.

[jakevdp]

The selection object only has x and y encodings, so there's not generally a good way to guess user intent. I'd prefer to make it easier for the user to be explicit.

I took a look: the problem is that zoom.x is already shorthand to refer to the x attribute of the selection object within expressions, so any attribute-based approach to this would potentially break existing code.