Data element visibility consistency

[stockiNail]

Developing an enhancement in chartjs-plugin-annotation, I have faced a weird situation about the visibility of the data elements. The new features is based on doughnut controllers and I have tested only there, therefore not sure if it's the same for all others.

I have seen that using hide and show API, when the dataIndex is passed as argument, the implementation is changing the data elements adding/setting the hidden option in each data element.
Instead, toggleDataVisibility and getDataVisibility API are managing an internal object, _hiddenIndices, which is not touch from hide and show.

In this way, if I'm performing an hide action to a specific index of a dataset, the getDataVisibility is returning always true (case with 1 dataset).

Not sure if it's working as designed.

Here a codepen, https://codepen.io/stockinail/pen/MWBjQjO, where clicking on chart area, the first element will be hidden but the getDataVisibility invocation returns true, see the console.