@spec append(vl :: VegaLite.t(), VegaLite.t() | [VegaLite.t()]) :: VegaLite.t()

Appends the given layer or layers to the input specification.

If no layer exists in the input spec, a new layer object will be added with the encoding and mark options of the input spec.

Raises if the input vl is not a single view or layered specification.

@spec prepend(vl :: VegaLite.t(), VegaLite.t() | [VegaLite.t()]) :: VegaLite.t()

Prepends the given layer or layers to the input specification.

If no layer exists in the input spec, a new layer object will be added with the encoding and mark options of the input spec.

Raises if the input vl is not a single view or layered specification.

@spec to_layered(vl :: VegaLite.t()) :: VegaLite.t()

Converts a single view plot to a layered plot.

encoding, mark and data will be extracted and added to a new layer.