* Early data parsing + stacking by value
* Review comments
* review comments
* Remove reduntant parsing
* Couple CC warnings
* Optimize filterBetween
* More migration info
`controllers.*.js` and `core.datasetController.js` are now importable (no more function export), that's why there is so many changes mainly due to one indentation level removed. Split code for `bar/horizontalBar` and `doughnut/pie` in separate files, added a global controllers import (`src/controllers/index.js`) and add tests to check that all dataset controllers are correctly registered under `chart.controllers.{type}`.
* added 'angle' option to polar charts. image comparison test is work in progress; not currently passing
* removed unnecessary variable assignment
* code cleanup based on PR; for 'angle' option on polarCharts
* Made polar chart image comparison test pass by removing debug flag. Also explicitly marked _computeAngle as private.
* Removed visibleCount computation in polar chart
* split out code related to updating the radius in polar chart's update function, into it's own 'updateRadius' function
* made updateRadius method private
* fix linting error
* updated polar charts to read custom angles from "chart.options.elements.arc.angle" instead of "chart.options.angle"
Default options can now be accessed by importing `core/core.defaults`. The returned object acts as a singleton and is populated when importing classes that expose their own default values (meaning that importing only `code.defaults` results in an empty object). Also make `Chart.Ticks` and `Chart.Interaction` importable since existing defaults rely on these values.
Add the `defaults._set` method that make easier declaring new defaults by merging given values with existing ones for a specific scope (`global`, `scale`, `bar`, etc).
Properly export helpers and remove dependencies to `Chart.helpers`. Helpers can now be accessed from `src/helpers/index.js` (`var helpers = require('path/to/helpers/index')`, instead of `var helpers = Chart.helpers`).
For consistency with `valueOrDefault`, `valueAtIndexOrDefault` now returns null if `value` (expected array) is null. Also get rid of the superfluous `get` prefix in `getValueOrDefault` and `getValueAtIndexOrDefault`.
gridLines.circular is a new option that toggles circular lines. This allows radar charts with circular lines #3082
pointLabels.display is a new option that toggles the display of point labels.
The existing angleLines.display is used with the new pointLabels.display setting is used to trigger the radar like settings.
This required changing the default polar area config.
This change allows for smaller minified code in the final version, resulting in a smaller file size. Some files had previously used _this, but that has been changed to me to keep consistency throughout the project.
Data controllers should now rarely implement addElements and addElementAndReset but instead should define dataElementType (and optionally datasetElementType). Also remove some dead code (e.g. numBars, colorForNewElement, etc.).
Fix access of uninitialized meta data while calculating circumference in the polar area chart by caching the number of visible elements in the update() method. Also make the calculateTotal() of the doughnut chart tolerant of uninitialized meta data.
New Chart.Element.hidden bool flag storing the visibility state of its associated data. Since elements belong to a specific chart, this change allows to manage data visibility per chart (e.g. when clicking the legend of some charts).
This commit also changes (fixes?) the polar chart animation when data visibility changes. Previous implementation was affected by an edge effect due to the use of NaN as hidden implementation.
Introduced a new meta.hidden 3 states flag (null|true|false) to be able to override dataset.hidden when interacting with the chart (i.e., true or false to ignore the dataset.hidden value). This is required in order to be able to correctly share dataset.hidden between multiple charts.
For example: 2 charts are sharing the same data and dataset.hidden is initially false: the dataset will be displayed on both charts because meta.hidden is null. If the user clicks the legend of the first chart, meta.hidden is changed to true and the dataset is only hidden on the first chart. If dataset.hidden changes, only the second chart will have the dataset visibility updated and that until the user click again on the first chart legend, switching the meta.hidden to null.
Meta info are now scoped by chart and moved under the dataset._meta map { chart.id -> meta }. Meta for a specific chart (and dataset) can be accessed using chart.getDatasetMeta(datasetIndex) or from the dataset controller using getMeta(). Note that helpers.uid() now generates an int (instead of a string) to make lookups in the _meta map faster.
