Support format expression function

[mattijn]

The following spec renders fine:

import altair as alt
url = 'https://gist.githubusercontent.com/mattijn/73efd6796132c997ae2dcad45e8b7873/raw/41512de3da5bbc372ac40d9ae18519a46ac5426c/doy_thresholds.csv'
chart = alt.Chart(url, height=10).mark_area(interpolate='monotone').encode(
    x=alt.X('doy:O', sort='ascending').bin(maxbins=20),
    y=alt.Y('sum(value):Q').axis(None).scale(range=[10, -10]),
    row=alt.Row('variable:O').title(None).header(labelAngle=0, labelAlign='left')
).configure_facet(
    spacing=0
).configure_view(
    stroke=None
)
chart

But when trying to extract the transformed data it errors:

chart.transformed_data()

ValueError: Pre-transform error: Requested variable (Variable { name: "source_0", namespace: Data }, [])
 requires transforms or signal expressions that are not yet supported

When looking to the spec, I'm not sure which transform is being utilised that is not yet supported.

[jonmmease]

Looking at the generated Vega spec, there is this transform

        {
          "type": "formula",
          "expr": "!isValid(datum[\"bin_maxbins_20_doy\"]) || !isFinite(+datum[\"bin_maxbins_20_doy\"]) ? \"null\" : format(datum[\"bin_maxbins_20_doy\"], \"\") + \" – \" + format(datum[\"bin_maxbins_20_doy_end\"], \"\")",
          "as": "bin_maxbins_20_doy_range"
        },

The issue is that the format expression function isn't implemented yet. I have an old Rust implementation of the Vega format function that I developed a while back for another project. I'll see if I can port that over to VegaFusion. Although, it may also be worth adding a special case for format(value, "") that just transforms this into toString(value), as this will be easier to support across SQL backends.

Also cross reference #336 for better error messages when something isn't supported.