How to set width of VegaLite.jl Plot at time of image saving?

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Hi, is there a way to set image width for VegaLite.jl at the time of image saving?

And maintaining the default chart ratio (sometimes when I set width the ratio is changed)?

using DataFrames, VegaLite, JSON

# Plotting code does not know the required image width, so it can't specify it
df = DataFrame(x=0:9, y=(i->i^2).(0:9))
vspec = (
  mark=(type=:line,),
  encoding=(
    x=(field="x", type=:quantitative),
    y=(field="y", type=:quantitative)
  )
)
spec = VegaLite.VLSpec(JSON.parse(JSON.json(vspec)))
fig = spec(df)

# Image saving code, it knows the image width, but how to specify it?
VegaLite.save("plot.png", fig)

Also, when using multi-plot plot (facet, column, row) - the image width multiplied by count of rows, so you specify width=800px and with 3 columns got 2400px instead. Is there a way to set width for the whole image?