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https://github.com/vijithassar/bisonica

accessible data visualizations with Vega Lite
https://github.com/vijithassar/bisonica

accessibility data-visualization vega vega-lite visualization

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accessible data visualizations with Vega Lite

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README 

        
# Overview



bisonica is a minimalist and accessibility-first alternative renderer for the open source [Vega Lite](https://vega.github.io/vega-lite/) standard for data visualization, a replacement for the [original `vega-lite.js` library](https://github.com/vega/vega-lite) which reads in the same [JSON configuration objects](https://vega.github.io/vega-lite/docs/) and outputs charts.



# Install



Install from [npm](https://www.npmjs.com/package/bisonica) using your tool of choice. For example:



```bash

npm install bisonica

```



# Quick Start



The `chart()` function takes a Vega Lite JSON specification object and uses it to create a chart rendering function which can be run using [`d3-selection`](https://github.com/d3/d3-selection).



```javascript

import { select } from 'd3';

import { chart } from 'bisonica';



const dimensions = {

    x: 500,

    y: 500

};



// create a chart rendering function

const renderer = chart(specification, dimensions);



// render the chart

renderer(select('div'));

```



Or the equivalent syntax with `selection.call()`:



```javascript

// render the chart

select('div').call(renderer);

```



You'll probably want to load the included stylesheet? Feel free to use your own alternative if you want, though.



You can load it directly:



```html



    



```



Or import with a packager or build tool:



```javascript

import "bisonica/styles.css";

```



# Why?



## Accessibility



When faced with an accessibility concern, bisonica typically just defaults to the most accessible option, whereas `vega-lite.js` might require more elaborate JSON in the specification object in order to achieve the same result. Some accessibility features such as the keyboard navigation and audio sonification cannot be replicated with the standard `vega-lite.js` renderer at all.



## Performance



bisonica is often considerably faster than rendering the same chart using `vega-lite.js`. This will depend on the specific chart configuration and the input data, but as an example, pie charts have been clocked rendering in as little as 1.25 milliseconds.



## Customization



Unlike `vega-lite.js`, bisonica renders legends as HTML next to the SVG instead of inside the SVG, and as a result they are much easier to restyle with CSS or even control with custom JavaScript behaviors.



# Comparison



In general, bisonica may be a good choice if you need to render straightforward charts with strong accessibility properties using Vega Lite's JSON as the backing format and you can handle writing the custom CSS that will probably be necessary to get the generated graphics over the finish line for your use case.



On the other hand, the standard `vega-lite.js` renderer is definitely still the way to go if you need its more elaborate graphical options, [faceted trellis plots](https://vega.github.io/vega-lite/docs/facet.html), charts which don't rely on custom styling, a dynamic exploratory workflow powered by [evaluating string expressions](https://github.com/vega/vega-expression), or any of the features bisonica has intentionally omitted.



## Omissions



bisonica is still a work in progress and as such supports only a subset of Vega Lite functionality. The supported chart forms are listed in [`marks.js`](./source/marks.js).



Data loading will not [parse inline strings](https://vega.github.io/vega-lite/docs/data.html#inline).



Nested fields must be looked up using dot notation (e.g. `datum.field`), not bracket notation (e.g. `datum['field']`).



[Predicates](https://vega.github.io/vega-lite/docs/predicate.html) defined with string expressions only support simple comparisons (e.g. `"datum.value < 100"` or `"datum.group === 'a'"`).



The [calculate transform](https://vega.github.io/vega-lite/docs/calculate.html) only supports deriving new fields with string concatenation and static functions but can't do arbitrary math. (If you need arbitrary math, do it in JavaScript and attach the results to your specification before rendering.)



Escaping special characters in field names is not supported. Instead, you should mutate your data before rendering to clean up the affected field names.



Advanced Vega Lite features like [`facet`](https://vega.github.io/vega-lite/docs/composition.html#faceting) and [`parameters`](https://vega.github.io/vega-lite/docs/parameter.html) are not yet available.



Rendering to alternative output formats such as `` instead of `` will most likely never be supported.



# Errors



## Throwing



Errors inside the chart library are nested, and error handling typically appends information to `error.message` and then re-throws the same error object. This naturally has the effect of augmenting the messages generated by the runtime with written explanations meant for humans at every layer of the stack, sort of a "semantic stack trace."



## Catching



To set a custom error handler, pass a function to the `.error()` method.



```javascript

// create a chart renderer

const renderer = chart(specification, dimensions)



// error handling function

const errorHandler = (error) => alert(error.message);



// attach the error handling function to the chart renderer

renderer.error(errorHandler);

```



By default, errors are handled at the top level with `console.error`, equivalent to `renderer.error(console.error)`.



## Disabling



To disable default trapping of errors and instead surface the semantic stack traces to the consumer:



```javascript

// disable catching and allow errors

// to propagate up to the caller

renderer.error(null);

```



You'll then want to handle these in your page or application.