README.md

Vega-Lite Library

This repo is a small personal library of useful visualization approaches with Vega-Lite. Basically, I needed to keep things in one place as I learned, experimented, and wound up settling or finding useful techniques. This is the place.

This repo uses:

• Vega-Lite for visualizations
• d3: specifically d3.csv to ingest data
• Arquero to ingest and manipulate data
• Various publicly-available data on Github, much of it from the NYC DOHMH coronavirus-data repository

Pages

This will probably get out of date pretty soon, but:

• Basic bar: from named data source: storing data outside the spec, inserting it with Vega View.
• Sorted bar: d3.csv, named data source, and a sorted bar chart.
• Line chart: d3.csv, named source, remote spec.
• Using vconcat
  • Small multiples: charting several series from a single source.
  • Enhancing with repeat - using repeat and a do-nothing selection.
  • Vconcat and layer. A lengthy spec, but good charts.
• Time Selection: using VL's brush to choose a range and display it on concatenated charts.
• Using Arquero
  • Basic line chart plus inserting via named data source.
  • Basic bar plus a transpose function
  • Using Arquero with Await: getting async stuff running.
• Arquero and time selection:
  • Full-or-90-day selection: Arquero import, 90-day calculation, and basic radio button toggle.
  • Set months view: with a reset button.
• Calculating within VL spec: this is the syntax for using parseFloat() to create and chart calculated fields. Also done here.
• Map stuff:
  • Map and bar, companion and colors.
  • Map with basic hover-highlight.
  • Map and bar with mutual hover-highlighting
• Converting VL spec to Vega spec should give us a platform to develop simpler Vega-Lite spec, convert, and add some functions unique to Vega.

Techniques

A lot of these are shown pretty simply and clearly in each page, but here's a quick summary.

Using a named data source

Using a named data source gives us more opportunities to manipulate the source of the data (that is, importing data into the browser, manipulating it using JavaScript, and delivering it to the chart - rather than relying on a precisely-formatted CSV).

Filtering by time in a file

Essentially we're using layer.transform as transform": [{"filter": "datum.date_of_interest > 0"} and then subbing out that 0 for a value we calculate. (Note that with a bar-and-line chart, having two layers, we need to have this filter on both layers.)

Calculating within a file

We can calculate in transform.calculate, but we need to use parseFloat() for it to work. The below does the trick, essentially creating a new calculated field of TOTAL_CASE_COUNT that we can use to chart.

"transform": [
 {"calculate": "parseFloat(datum.CASE_COUNT) + parseFloat(datum.PROBABLE_CASE_COUNT)", "as": "TOTAL_CASE_COUNT"}
 ],

Using Arquero

A lot of these examples bring data in using Arquero. This plus using named data sources should let us do a lot more manipulation within the browser.

Map things

• Map with highlight-on-hover
• Map and bar

Other layout stuff

• Sorted bar charts
• Vertical concatenation (useful for small multiples)

To do list

• Continue documentation
• Double-bounded range slider
• Template out stuff like:
  • bar with highlight (in progress)
  • small multiples with mutual highlighting
  • map-and-bar with mutual highlighting
  • populate a selection menu using Arquero's dt.columns()