feat(vega): implement transform for vega

[rgbkrk]

This enables the ability to use vega and vega-lite, either through Altair, ipyvega, or generally with the right vega(-lite) mimetype (application/vnd.vega+json and application/vnd.vegalite+json).

Items to follow up with:

• Handling height and width
• Dealing with the external links (shown above)
• Tests!

/cc @ellisonbg

[codecov-io]

Current coverage is 88.23% (diff: 100%)

Merging #1113 into master will increase coverage by 0.18%

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+ Hits           1260       1282    +22   
  Misses          171        171          
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Powered by Codecov. Last update 50bb758...a75b8a9

[rgbkrk]

[rgbkrk]

Altair's declarative syntax makes this all just too easy. ❤️ it.

[rgbkrk]

Note: until ipyvega exports the vega mimetypes used above, folks have to wrap altair objects with the little vegify function I wrote:

import IPython.display
def vegify(spec):
    IPython.display.display({
        'application/vnd.vegalite+json': spec.to_dict()
    }, raw=True)

v = vegify

[captainsafia]

Looks good!

[lgeiger]

Webpack throws the following error on my machine:

ERROR in ./app/~/vega-scenegraph/src/util/canvas.js
Module not found: Error: Cannot resolve module 'canvas' in /Users/lukasgeiger/code/nteract/app/node_modules/vega-scenegraph/src/util
 @ ./app/~/vega-scenegraph/src/util/canvas.js 11:17-34

ERROR in ./app/~/vega-scenegraph/src/util/ImageLoader.js
Module not found: Error: Cannot resolve module 'canvas' in /Users/lukasgeiger/code/nteract/app/node_modules/vega-scenegraph/src/util
 @ ./app/~/vega-scenegraph/src/util/ImageLoader.js 51:19-36

[rgbkrk]

Does it finish building for you though? I can add an escape for that, as we have native browser canvas as soon as it's in the Browser Window.

[lgeiger]

Yes it builds fine

[lock]

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