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hrbrmstr / vegalite

R ggplot2 "bindings" for Vega-Lite
http://rud.is/b/2016/02/27/create-vega-lite-specs-widgets-with-the-vegalite-package/
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support of "layers" specification (multiple marks in one chart) #16

Open TomBor opened 7 years ago

TomBor commented 7 years ago

It would be really great to have "layers" in vegalite as this example on stackoverflow show it. Thanks

jsonbecker commented 7 years ago

I'm not seeing great specs on Vega-Lite for layer... if we knew what are the required and allowed fields it shouldn't be too bad to implement.

Do you have any ideas for the API? That's a far more difficult question IMO.

TomBor commented 7 years ago

layers works in vega-editor, so I suppose it's not specific to future 2.0. But I can't find any specs for layers except in this pull request. I don't know if it help you.

jsonbecker commented 7 years ago

It looks like layers are an array of mark and encoding pairs. If so, maybe the right way to do this would be something like:

vegalite() %>%
add_data(...) %>%
add_layer(mark = mark_bar(), encoding = list(encode_x(), encode_y())) %>%
add_layer(mark = mark_point(), encoding = list(...)) %>%
...

I'm a little worried without more docs that it would be easy to make invalid JSON out of this, but I could see it being possible. Note, I'd probably have add_layer pack any existing mark and encoding elements into a layer...

jsonbecker commented 7 years ago

Another thought:

You could add a layer property that defaults to 1 for each property that can be in a layer. Then you can have add_layer() append to the layer list, layer_count() tell you how many layers an object has, show_layers() reply with the list saved to layer. Then, to_spec, print, etc that evaluates it can have a check for layer_count() == 1 and basically remove the layer list in that case and unnest it.

That'd be a pretty big change though.

kenklin commented 7 years ago

https://idl.cs.washington.edu/files/2017-VegaLite-InfoVis.pdf - 3.2 View Composition Algebra Describes layers, hconcat/vconcat, and facet but I don't see these descriptions on the Vega-Lite site, other than faceting through row or column channels.

TomBor commented 7 years ago

Vega-lite 2.0 documentation is on line but not complete. Specifications for layers will be here : https://vega.github.io/vega-lite/docs/layer.html