[META] Use ES6 constructs, idioms in metrics-graphics

[wlach]

Now that we're using babel to transpile, we should start using some ES6 idioms. It might be a good idea to use an automated-converter like lebab to automate this process.

For now we should try to do this in a backwards-compatible way. i.e. none of the public interfaces provided by metricsgraphics should change at all.

@lobax and some students might be interested in taking these on.

[lobax]

A little wrap up of what we did and what is left to do:

All files under charts except table.js have been refactored to use ES6 with lebab. There are currently no unit tests for table.js, which is why we didn't refactor it. Maybe a few should be added before further refactoring is preformed?

The files under charts could also further be refactored to use ES6 Classes.

[garygreen]

Once this is done, is it worth changing the main in package.json to point to the main entry point for MG? Reason being, there is pretty decent support for ES6 now in all major browsers and it would allow Webpack / Rollup users to allow their bundler to bundle MG to create smaller and faster builds, targeting whatever browser they wish to support. See https://philipwalton.com/articles/deploying-es2015-code-in-production-today/ for more info. Google's autotrack is pushing this too see googleanalytics/autotrack#137 for more info.

Also, it would be great if you could include only the charts you use:

import 'metric-graphics/charts/line';

Or tree shaking:

import { lineCharts, mg } from 'metric-graphics';

So any other charts / modules not used by metric graphics will be removed (at least by Webpack, and Rollup I believe)

Now to cut down the size of the d3 dependency.... but that's another topic 😄

[edmorley]

Once this is done, is it worth changing the main in package.json to point to the main entry point for MG?

There's a safer intermediate step: leave main pointing at an ES5 version, and use module for the ES2015 version. Webpack et al know to prioritise the module reference over main.