Reduce impact of metrics-graphics on bundle size

[edmorley]

Hi :-)

The README currently says:

MetricsGraphics.js is a library optimized for visualizing and laying out time-series data. At under 80KB (minified), ...

However it actually causes a 385KB (minified) increase in bundle size:
https://bundlephobia.com/result?p=metrics-graphics@2.15.6

This is larger than react+react-dom (96KB), jQuery (85KB), lodash (69KB), bootstrap (49KB) and flot (32KB)
...all added together! (And I don't think this is even including the CSS)

Whilst 70% of the 385KB comes from the dependencies of metrics-graphics - the weight they add can't just be written off when discussing its size, since it's the way that metrics-graphics is structured/bundled that causes all of them to be included even if they are not used.

It's also worth noting that d3 got quite a bit larger in v5 (compare v4 and v5) which is not included in the above (since the v5 upgrade is only on master) - so the issue is going to be even worse in the next release / over time as the d3 package ecosystem gets larger.

Things that would likely help with this:

• Using ES6 imports/exports and not globals (part of [META] Use ES6 constructs, idioms in metrics-graphics #811)
• Switching to rollup (Switch from gulp to rollup #902)
• Depending on/importing the individual d3 packages rather than the overall d3 package (eg Use subset of d3 v4 modules #662 / Use subset of d3 v4 modules #667)
• Making both a CJS and an ES6 modules version of the package available (the former under main in package.json, the latter under module)
• Seeing if "sideEffects": "false" can be used for improved tree shakability: https://webpack.js.org/guides/tree-shaking/#mark-the-file-as-side-effect-free

[jens-ox]

Hi! Please see #929 -- the new version comes at ~10kB minified and gzipped.