Add a "show in legend" toggle for items to hide them from the legend

[s-nie]

Document what "name" is for and add a show_in_legend toggle to hide items in the legend.

As discussed in #82 (comment).

• I have followed the instructions in the PR template

[oscargus]

Would it make sense to have an Option<String> for name and setting None if you do not want to have a legend entry? Or will that lead to the same id for all None items and therefore the same properties?

Or maybe put another way: what is the benefit of being able to toggle legend visibility separately, rather than just setting the name to an empty String? (Or more explicitly, None per above.) Not saying that there are no, just want to understand the difference.

Edit: I read up a bit in #82, but I still cannot really get it. Wasn't it possible to just pass an empty String earlier?

[s-nie]

Would it make sense to have an Option<String> for name and setting None if you do not want to have a legend entry? Or will that lead to the same id for all None items and therefore the same properties?

That was pretty much the behavior before #82, and I preferred that as well. @Wumpf my understanding is that this limits visibility checking as the name is also used for that?

[Wumpf]

Or will that lead to the same id for all None items and therefore the same properties?

that's exactly the issue, yes. You could still provide an id explicitly, but accepting None would make this very non-intuitive.
Guess we could still hide empty strings in the legend (does it do that?) but then you have to add an id explicitly.

[Wumpf]

should check if empty names show up and update doc accordingly

[Wumpf]

well that's not entirely true though, it's the id that tracks all this. It's just that by default the id is created from that name.

Suggested change

	/// The given name will show up in the legend and serves to track the visibility of items.
	/// Multiple items may share the same name, in which case they also share a legend entry.
	/// The given name is used to create the item's id and will show up in the legend.
	///
	/// Multiple items may share the same name (and id), in which case they also share a legend entry.
	/// The item's id is used to for visibility tracking and to refer to items in the plot's response.

should probably expand doc on the id setter as well.