export_png failing for group_by plots

[barryrowlingson]

This works

  > p=mtcars %>% 
     ggvis(~wt, ~mpg) %>% 
     layer_points(fill = ~factor(cyl))
  > export_png(p,"plot1.png")
  > 

But this fails

 > p=mtcars %>% 
    ggvis(~wt, ~mpg, fill = ~factor(cyl)) %>% 
    layer_points() %>% 
    group_by(cyl) %>% 
    layer_model_predictions(model = "lm")
> export_png(p,"plot1.png")
 /usr/local/lib/node_modules/vega/vega.js:4799
   var tx = vg.data[def.type]();
                             ^
 TypeError: Property 'treefacet' of object #<Object> is not a function
     at vg.parse.transform (/usr/local/lib/node_modules/vega/vega.js:4799:29)
     at Array.map (native)
     at Object.vg.parse.dataflow (/usr/local/lib/node_modules/vega/vega.js:4261:34)
     at /usr/local/lib/node_modules/vega/vega.js:4234:27
     at Array.forEach (native)
     at Object.vg.parse.data (/usr/local/lib/node_modules/vega/vega.js:4223:16)
     at parse (/usr/local/lib/node_modules/vega/vega.js:4788:22)
     at Object.vg.parse.spec (/usr/local/lib/node_modules/vega/vega.js:4794:23)
     at Object.vg.headless.render (/usr/local/lib/node_modules/vega/vega.js:7248:12)
     at render (/usr/local/lib/node_modules/vega/bin/vg2png:55:15)

R 3.1.1, ggvis 0.4, and the following node modules:

    [email protected] /usr/local/lib/node_modules/vega
    ├── [email protected] ([email protected], [email protected])
    ├── [email protected]
    ├── [email protected] ([email protected], [email protected], [email protected], [email protected])
    ├── [email protected] ([email protected])
    └── [email protected] ([email protected])

A similar error has been reported on the ggvis google group but I thought it should go here.

[lucasxteixeira]

I have also commented this on the ggvis google group. I'm facing the exact same issue when trying to export a simple boxplot. Moreover, any grouping does not work.

[skanskan]

Where can I get vg2png or Vega for Windows?