`Only chart objects can be used in ConcatChart` error when `concat` subcharts

As per https://github.com/altair-viz/altair/issues/1925#issuecomment-669238407 when I try to horizontally alt.concat(three_subcharts), Altair raises ValueError: Only chart objects can be used in ConcatChart. I find this error confusing, because each of the three subcharts do individually plot correctly.

Code:

    brush = alt.selection(type='interval', resolve='global')
    three_subcharts = []
    hyperparams = [('Learning Rate', "log"), ('L2 Penalty', "log"), ('Dropout', "linear")]

    for hyperparam, scaling in hyperparams:
        color = alt.condition(brush, alt.ColorValue('red'), alt.ColorValue('gray'))

        chart = alt.Chart(data).mark_circle().encode(
            alt.X(hyperparam, type='quantitative', scale=alt.Scale(type=scaling)),
            alt.Y("Score", type='quantitative'),
            row=alt.Row("Metric", type='nominal'),
            color=color,
            tooltip=['Learning Rate', 'L2 Penalty', 'Dropout']
        ).add_selection(
           brush
        )
        three_subcharts.append(chart)

    print(three_subcharts)  # prints: [alt.Chart(...), alt.Chart(...), alt.Chart(...)] (each plots correctly as well)
    alt.concat(three_subcharts)  # ValueError: Only chart objects can be used in ConcatChart.

Data is in long format:

Dropout	Learning Rate	L2 Penalty	Metric	Score

[jakevdp]

alt.concat does not accept a single argument containing a sequence; it accepts multiple arguments each of which is a chart. Try this instead:

alt.concat(three_subcharts[0], three_subcharts[1], three_subcharts[2])

or, equivalently,

alt.concat(*three_subcharts)

[crypdick]

Thanks for the update @joelostblom!