Merge pull request #193 from tkf/setter

Document property accessors

Author: davidanthoff

docs/src/userguide/vlspec.md

@@ -104,6 +104,30 @@ The resulting `VLSpec` object is indistinguishable from one that is created via
 
 The main benefit of this approach is that one can directly leverage JSON vega-lite examples and code.
 
+## Manipulating `VLSpec` object directly
+
+Vega-Lite properties can be directly accessed as properties of the `VLSpec` object.
+
+```julia
+julia> spec = [(a=x, b=exp(x), c=sin(x)) for x in 0:10] |>
+           @vlplot(:point, x=:a, y=:b);
+
+julia> spec.mark
+:point
+
+julia> spec.encoding.x.field
+"a"
+```
+
+To modify properties, use [Setfield.jl](https://github.com/jw3126/Setfield.jl):
+
+```julia
+julia> using Setfield  # imports `@set` etc.
+
+julia> spec2 = @set spec.mark = :line
+       spec3 = @set spec2.encoding.y.field = "c"
+```
+
 ## Loading and saving vega-lite specifications
 
 The `load` and `save` functions can be used to load and save vega-lite specifications to and from disc. The following example loads a vega-lite specification from a file named `myfigure.vegalite`: