emacs-vega-view

This is a small library meant to facilitate exploratory data visualization using Vega from within emacs.

Installation

In order to use this package, you must have the Vega command line tools on your path. The easiest way to arrange that is to install the latest version with using npm:

npm install -g vega vega-lite vega-cli

If you need PNG support (for example, if your emacs hasn't been compiled with support for SVG rendering), you will also need to install the canvas package:

npm install -g canvas

Configuration

By default this library produces SVG output when used from an emacs that supports SVG. If you would prefer that it produce PNG -- for example, if your emacs has trouble displaying SVGs or the drawings you are producing would be very, very large as SVG (tens of thousands of embedded data points) -- you can set the var vega-view-prefer-png to any truth-y value to prefer PNGs:

(setq vega-view-prefer-png t)

In addition, view-vega defaults to using the current working directory of the buffer in which it is invoked. One can override by setting the var vega-view-base-directory to a specific directory. This can be useful if your specification refers to static assets using a relative path. Setting vega-view-base-directory back to a false-y value will return the default behavior.

Usage

Vega-view currently supports a single interactive function, vega-view, that can be invoked within a top-level form to visualize it as a Vega plot. Currently, three kinds of Vega notation are supported:

• JSON, which is passed directly to Vega.
• elisp, which is evaluated and converted to JSON before being passed to Vega. The elisp Vega specification format is, not coincidentally, the same as what is produced by called read-json on any Vega JSON specification.
• clojure, which is evaluated in the buffer's current cider context and converted to JSON before being passed to Vega. The clojure specification format is whatever EDN would translate into the JSON specification you want. This is, also not coincidentally, the same format one would use with Oz.

When vega-view is invoked it first identifies the preceding sexp (whatever that means for the language of the buffer), performs the mode-specific conversion described above, then pipes it through the Vega command line tools to convert the specification to an SVG drawing. The drawing -- or the errors produced by Vega while trying to produce it -- are then displayed in an image-mode buffer called *vega*. (Note that you can toggle between viewing an SVG image in an image-mode buffer as image or text using C-c C-c in that buffer.)

JSON

Suppose you have a json-mode buffer containing this Vega specification:

{
    "data": {
        "values": [
            {"a": "A", "b": 28}, {"a": "B", "b": 55}, {"a": "C", "b": 43},
            {"a": "D", "b": 91}, {"a": "E", "b": 81}, {"a": "F", "b": 53},
            {"a": "G", "b": 19}, {"a": "H", "b": 87}, {"a": "I", "b": 52}
        ]
    },
    "mark": "bar",
    "encoding": {
        "x": {"field": "a", "type": "ordinal", "axis": {"labelAngle": 0}},
        "y": {"field": "b", "type": "quantitative"}
    }
}

Placing the cursor after the final } and invoking vega-view will bring up a new window (in the emacs sense of the term) containing an SVG drawing made from this spec.

elisp

The code sample below was produced by invoking pp-eval-last-sexp after (json-read-file "sample.json") in the *scratch* buffer. The contents of sample.json are the same as in the JSON example above.

If we place the cursor after the final parenthesis of this elisp form, it will show the same drawing that was generated above.

'(($schema . "https://vega.github.io/schema/vega-lite/v4.json")
  (description . "A simple bar chart with embedded data.")
  (data
   (values . [((a . "A") (b . 28)) ((a . "B") (b . 55))
              ((a . "C") (b . 43)) ((a . "D") (b . 91))
              ((a . "E") (b . 81)) ((a . "F") (b . 53))
              ((a . "G") (b . 19)) ((a . "H") (b . 87))
              ((a . "I") (b . 52))]))
  (mark . "bar")
  (encoding
   (x (field . "a")
      (type . "ordinal")
      (axis (labelAngle . 0)))
   (y (field . "b")
      (type . "quantitative"))))

Note that, because the elisp code is evaluated before being sent to Vega, one can produce programmatic graphs easily using only elisp. For example, this code will thread in the contents of a JSON data file to another example specification to plot the chart that follows:

`(($schema . "https://vega.github.io/schema/vega-lite/v4.json")
  (data
   (values . ,(json-read-file "/Users/jack/src/emacs-vega/data/movies.json")))
  (transform . [((filter (and . [((field . "IMDB_Rating")
                                  (valid . t))
                                 ((field . "Rotten_Tomatoes_Rating")
                                  (valid . t))])))])
  (mark . "rect")
  (width . 800)
  (height . 600)
  (encoding
   (x (bin (maxbins . 60))
      (field . "IMDB_Rating")
      (type . "quantitative"))
   (y (bin (maxbins . 40))
      (field . "Rotten_Tomatoes_Rating")
      (type . "quantitative"))
   (color (aggregate . "count")
          (type . "quantitative")))
  (config (view (stroke . "transparent"))))

clojure

Just as in the case of elisp, one can write whatever clojure code they prefer and see the result of evaluating it and passing it through Vega (note that cider must be active!). For example, this form will plot twenty random values as a line chart:

{:data {:values (map hash-map
                     (repeat :a)
                     (range 1 20)
                     (repeat :b)
                     (repeatedly #(* 100 (Math/random))))}
   :mark "line",
   :width 800
   :height 600
   :encoding {:x {:field :a, :type "ordinal", :axis {"labelAngle" 0}},
              :y {:field :b, :type "quantitative"}}}

TODO

The vega-view function should be split into two functions, one that produces a drawing in a similar manner to eval-preceding-sexp (as the current function does) and one that works like eval-defun (converting the top level sexp).

Likewise, there should be an easy way to select between Vega-lite and full Vega. Perhaps by checking for the presence of a full Vega schema declaration?

We currently (set! *print-length* nil) in the cider handler to avoid truncation of the return value from the user's clojure REPL. This is rude, and we should find a better way.