Symbols with brackets do not plot

[sethaxen]

When using a type that implements the TableIterators interface, it seems that columns with brackets in their names, though valid symbols, are not processed correctly. While the column names are correctly assigned to their axes, none of the data is plotted. A minimal example:

julia> using VegaLite, DataFrames

julia> df = DataFrames.DataFrame([1:5, 6:10, 11:15], [:var1, :var2, Symbol("var[3]")])
5×3 DataFrame
│ Row │ var1  │ var2  │ var[3] │
│     │ Int64 │ Int64 │ Int64  │
├─────┼───────┼───────┼────────┤
│ 1   │ 1     │ 6     │ 11     │
│ 2   │ 2     │ 7     │ 12     │
│ 3   │ 3     │ 8     │ 13     │
│ 4   │ 4     │ 9     │ 14     │
│ 5   │ 5     │ 10    │ 15     │

julia> @vlplot(:point, data=df, x=:var1, y=:var2)

julia> @vlplot(:point, data=df, x=:var1, y=Symbol("var[3]"))

[oheil]

related discussion but with dot syntax:
https://discourse.julialang.org/t/how-to-use-strings-to-specify-plot-axes-in-vegalite-jl/67405
related VegaLite documentation:
https://vega.github.io/vega-lite/docs/encoding.html#field-def