Visualising data in Lisp-Stat

Lisp-Stat can render plots with text or Vega-Lite. Vega-Lite (VL) is a browser based plotting system based on a grammar of graphics language.

Plotting with text

Lisp-Stat includes text based plotting functions that are useful for visualising data in the REPL. cl-spark provides this functionality. The text/histogram function provides text based histograms. See that function for documentation.

Plotting with Vega-Lite

Although Vega-Lite can render plots in any browser that supports JavaScript, we found that the easiest integration is with Chrome, and we assume here this browser is available. It would work equally well in Electron, should someone want to pick up that integration.

Configuring a browser

You can configure a default browser in the file browser.lisp in the main system directory. The default is configured for Chrome, and this is the recommended browser. Browser command-line options can also be configured here.

Vega-Lite specification

Vega-Lite plots are specified with JSON to encoding mappings from data to the properties of the plot. In Lisp-Stat, the encodings are specified as ALISTs, and then transformed to Vega-Lite format with a JSON library. An ALIST is a convenient format, since this data structure is built-in to Common Lisp and therefore can be manipulated with standard functions.

The easiest way to see how a Lisp-Stat plot encoding looks is to decode one of the Vega-Lite examples. For example a simple bar chart from the JSON spec files looks like this in JSON:

  "$schema": "",
  "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": "nominal", "axis": {"labelAngle": 0}},
    "y": {"field": "b", "type": "quantitative"}

and and if we decode this with yason, using:

  (dex:get "" :want-stream t)
  :object-as :alist
  :json-arrays-as-vectors t))

we get:

(("$schema" . "")
 ("description" . "A simple bar chart with embedded data.")
   . #((("b" . 28) ("a" . "A")) (("b" . 55) ("a" . "B"))
       (("b" . 43) ("a" . "C")) (("b" . 91) ("a" . "D"))
       (("b" . 81) ("a" . "E")) (("b" . 53) ("a" . "F"))
       (("b" . 19) ("a" . "G")) (("b" . 87) ("a" . "H"))
       (("b" . 52) ("a" . "I")))))
 ("mark" . "bar")
 ("encoding" ("y" ("type" . "quantitative") ("field" . "b"))
  ("x" ("axis" ("labelAngle" . 0)) ("type" . "nominal") ("field" . "a"))))

We can encode this alist back to the original JSON with:

(let ((yason:*list-encoder* 'yason:encode-alist))
  (yason:with-output-to-string* ()
    (yason:encode *)))

This mechanism is generic, and as you will see, we can build up an alist that corresponds to any Vega-Lite spec by manipulating the values in the alist. This is what the convenience functions (like bar-chart) in the vglt package do. Most of the time you will be working with the convenience functions.

Manipulating the spec

Let’s suppose that the width of the chart is too narrow. The Vega-Lite documentation page for customizing size tells us that adding a ‘width’ property will let us control this. For this, simply push the property onto the spec. Assuming that you have saved the specification into a variable named *plot*:

(pushnew '("width" . 300) *plot*)

and you are done. Sometimes the value you wish to manipulate is a bit deeper in the specification property hierarchy. For these cases you can use the access system, which provides a convenient mechanism to access these nested values. Say, for example, you wanted to add an ordering to the bar chart. To sort by another encoding channel, you need to add a ‘sort’ property to one of the channels. If we want to sort x by the value of the y field:

(pushnew '("sort" . "-y") (accesses *plot* :encoding :x))

You can use Common Lisp functions to retrieve or set values within the alist just like you would any other list to build up the plot specification.

Adding data

There are two ways to plot Lisp-Stat data in Vega-Lite:

  1. embed the data into the specification
  2. write the data to a file and use a data URL

Embedding data

To embed the data into the plot specification, use the vglt:df-to-alist function. This will transform a data frame into an alist format that can be embedded into the Vega-Lite specification. For example, let’s start with an empty variable spec, with only a schema in it. Here is how you would add data to it from a data-frame:

(setf spec (acons "data" `(("values" . ,(df-to-alist data-frame))) spec))

Writing data

For larger data sets, you probably want to save the data to a file or network location and use the Vega-Lite ‘url’ property in the specification. You can write data frames to streams or strings in Vega-Lite format using the vglt:df-to-vl function. You can also use the inverse of this function: vglt:vl-to-df to read a Vega-Lite data array into a data-frame. This is useful for obtaining sample data sets from the Vega-Lite ecosystem.

Rendering the plot

There are two steps to rendering a plot:

  • saving the specification to a file in HTML and JavaScript format
  • calling the browser to render the plot

The first step uses a back-end specific function. For example the Vega-Lite function for saving a plot is vglt:save-plot, the Plotly one (when available), would be plty:save-plot. The browser functionality is common across all backends that use a browser for rendering, and these are located in the plot package.

This example demonstrates rendering data from the Lisp-Stat notebook on categorical variables. First some quick boilerplate to set up the environment:

(ql:quickload :ips)         ; data examples
(ql:quickload :plot/vglt)   ; Vega-Lite plotting
(in-package :ips)
(defparameter online (read-csv (dex:get ips::eg01-07 :want-stream t)))
(defparameter online-bar-chart (vglt:bar-chart online "SOURCE" "COUNT"))

Now we can render the spec like so:

(plot:plot-from-file			        ; Common browser plotting
 (vglt:save-plot 'online-bar-chart))	; Vega-Lite specific save

You should see a new Chrome window containing the plot.