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.
Dataset effective type is now stored under meta.type, allowing many charts to share the same dataset but with different types. Also move dataset.bar flag to meta.bar.
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.
