# elm-vega

![alt text](https://raw.githubusercontent.com/gicentre/elm-vega/master/images/banner.jpg "elm-vega")

_Declarative visualization for Elm_

This library allows you to create Vega-Lite specifications in Elm providing a pure functional interface to declarative visualization construction.

The library does not generate graphical output directly, but instead it allows you to create a JSON _specification_ that can be sent to the Vega-Lite runtime to create the output.
This is therefore a 'pure' Elm package without any external non-Elm dependencies.

## Example

A simple scatterplot encoding engine power and efficiency as x- and y-position and country of origin with colour:

```elm
let
    cars =
        dataFromUrl "data/cars.json" []

    enc =
        encoding
            << position X [ PName "Horsepower", PmType Quantitative ]
            << position Y [ PName "Miles_per_Gallon", PmType Quantitative ]
            << color [ MName "Origin", MmType Nominal ]
in
toVegaLite [ cars, mark Circle [], enc [] ]
```

This generates a JSON specification that when sent to the Vega-Lite runtime produces the following output:

![alt text](https://raw.githubusercontent.com/gicentre/elm-vega/master/images/simpleScatterplot.png "Simple scatterplot")

The specification generated by elm-vega for this example looks like this:

```javascript
{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "data": {
    "url": "data/cars.json",
    "format": {
      "type": "json"
    }
  },
  "mark": "circle",
  "encoding": {
    "x": {
      "field": "Horsepower",
      "type": "quantitative"
    },
    "y": {
      "field": "Miles_per_Gallon",
      "type": "quantitative"
    },
    "color": {
      "field": "Origin",
      "type": "nominal"
    }
  }
}
```

## Why elm-vega?

### A rationale for Elm programmers

There is a [demand for good visualization packages](https://github.com/elm-lang/projects#data-visualization) with Elm.
And there are certainly plenty of data visualization packages available, ranging from low level [SVG rendering](http://package.elm-lang.org/packages/elm-lang/svg/latest) through a host of charting packages (e.g. [Charty](http://package.elm-lang.org/packages/juanedi/charty/latest) and [elm-charts](http://package.elm-lang.org/packages/simonh1000/elm-charts/latest)) to elegant, [opinionated chart construction](http://package.elm-lang.org/packages/terezka/elm-plot/latest) and more [comprehensive visualization libraries](http://package.elm-lang.org/packages/gampleman/elm-visualization/latest).
The designs of each reflects a trade-off between concise expression, generalisability and comprehensive functionality.

Despite the numbers of libraries, there is a space for a higher level data visualization package (avoiding, for example the need for explicit construction of chart axes) but one that offers the flexibility to create a wide range data visualization types and styles.
In particular no existing libraries offer easily constructed interaction and view composition (building 'dashboards' comprising many chart types).
elm-vega is designed to fill that gap.

**Characteristics of elm-vega**

-   Built upon the widely used [Vega-Lite](https://vega.github.io) specification that has an academic robustness and momentum behind its development (Vega-Lite is itself built upon the hugely influential [Grammar of Graphics](http://www.springer.com/gb/book/9780387245447)).

-   High-level declarative specification (a chart can be fully specified in as few as five lines of code)

-   Extensive use of _union types_ means "the compiler is your friend" when building and debugging complex visualizations.

-   Flexible interaction for selecting, filtering and zooming built-in to the specification.

-   Hierarchical view composition allows complex visualization dashboards to be built from trees of simpler views.

### A rationale for data visualisers

[Vega-Lite](https://vega.github.io) hits the sweet spot of abstraction between lower-level specifications such as [D3](https://d3js.org) and higher level visualization software such as [Tableau](https://www.tableau.com).
By using JSON to fully encode a visualization specification Vega-Lite is portable across a range of platforms and sits well in the JavaScript / Web ecosystem.
Yet JSON is really an interchange format rather than one suited directly for visualization design and construction.

By wrapping Vega-Lite within the Elm language, we can avoid working with JSON directly and instead take advantage of a typed functional programming environment for improved error support and customisation.
 This greatly improves reusability of code (for example, it is easy to create custom chart types such as box-and-whisker plots that can be used with a range of datasets) and integration with larger programming projects.

Elm and elm-vega provide an ideal environment for educators wishing to teach Data Visualization combining the beginner-friendly design of Elm with the robust and theoretically underpinned design of a grammar of graphics.

## Limitations

-   elm-vega does not render graphics directly, but instead generates data visualization specifications that may be passed to JavaScript for rendering.

-   elm-vega is currently limited to the Vega-Lite specification (and a small amount of the Vega specification) so is limited to Cartesian coordinate systems (e.g. no pie charts or rose diagrams).
    However, as Vega-Lite develops (e.g. geospatial mapping will be available shortly), so will elm-vega.
    And the door remains open for full Vega integration in the future.

-   While limited animation is possible through interaction and dynamic data generation, there is no direct support for animated transitions (unlike D3 for example).

## Further Reading

-   To get started with your first Elm-Vega program, see [Creating your first Elm-Vega visualization](docs/helloWorld/README.md).
-   For a more thorough set of examples/tutorial, see the [elm-vega walkthrough](docs/walkthrough/README.md).
-   For the elm-vega API documentation drop [documentation.json](docs/api/documentation.json) in the [Elm doc preview page](http://package.elm-lang.org/help/docs-preview)
-   Further examples can be found in the [elm-vega vlExamples folder](vlExamples/)