Altair: Possible to use the slider values in transform_calculate?

[KODeKarnage]

Forum would be a better place for this question, sorry, but I couldn't find one yet :)

From what I can see, the slider can only control selection, is that correct?

I want to be able to give my users the option to scale the chart within a certain range, but would prefer not to have to pre-calculate the results for each point in that range.

If I could get the slider position, I could use that in a transform_calculate to do exactly what I need and reduce the size of the html by 50 times!

[jakevdp]

No, I don't think that is possible. Unless I'm mistaken, Vega-Lite can only bind slider values to selections, which cannot be used to set arbitrary chart properties.

I believe you can do something along those lines with Vega, though. It might be worth asking on the vega slack channel.

[breadbaron]

Along the same lines - is it possible to use a slider's values to select subsets of data?

E.g. I'd like to make a plot like this, but instead of only selecting the year selected in the slider, select all years >= that value:

https://altair-viz.github.io/gallery/us_population_over_time.html?highlight=slider

[adrielvieira]

Did you managed to do that @breadbaron?

[breadbaron]

@adrielvieira no I don't believe so

[adrielvieira]

I managed to change the color based on the value of the slider, following your mention of value comparison:

slider = altair.binding_range(min=0, max=100, step=1)
selector = altair.selection_single(name="SelectorName", fields=['field_name'],
                                   bind=slider, init={'score': 100})

altair.Chart(dataframe).mark_bar().encode(
    y='other_field_name',
    x='field_name',
    color=altair.condition('datum.field_name< SelectorName_ field_name', altair.value('lightblue'), alt.value('lightgray')),
).add_selection(selector)

You can use the same logic to filter data instead of change color:

slider = altair.binding_range(min=0, max=100, step=1)
selector = altair.selection_single(name="SelectorName", fields=['field_name'],
                                   bind=slider, init={'score': 100})

altair.Chart(dataframe).mark_bar().encode(
    y='other_field_name', 
    x='field_name').add_selection(
    selector
).transform_filter(
    'datum.field_name < SelectorName_ field_name'
)

[jakevdp]

Interesting... didn't know this was possible!

It might be useful to think about adding a high-level syntax for doing this; e.g.

chart.transform_filter(
   alt.datum.field_name < slider.value
)

or something along those lines, so that users don't have to worry about internal details surrounding names & formats.

[breadbaron]

Wow, cool @adrielvieira !