Selected lines in Line plot should show metadata

**What's your use case?**

I render a Line plot with aggregated data for a number of countries. The country names are metadata strings. Nothing happens if I hover over any specific line, well OK, but even when I select a specific line and this is indicated by fattening the line (by default) I don’t get any label.

**What's your proposed solution?**

I would like for selected lines to show their metadata in a similar manner to how Scatterplots show their metadata when hovering the mouse over them, or at the very least when explicitly selecting the line.

**Are there any alternative solutions?**

Well, the alternative is not to display the data, but that is a bit sad, innit?

Start by locating the Line plot and Scatterplot implementations and comparing how selection and metadata display are handled. Confirm the expected behavior for a selected line, then verify that its country metadata is shown consistently with Scatterplots.

Written by the indexing model from the issue text.