[Feature] Hyperlink/anchor support for pie charts

### What problem does this feature solve?

Clicking a pie chart section or a legend entry should be able to take the user to a specified URL. I render my SVGs on the server, and browser Javascript is not strictly necessary for this feature, I currently achieve it by wrapping the sections and labels in `…`.

This potentially could be extended to some other chart types.

### What does the proposed API look like?

Not having much eCharts experience, I can only propose a suboptimal API. In my case it looks like this:

```json
{
"dataset": { "source": [["Company 1", 60, "/companies/1"], ["Company 2", 40, "/companies/2"]] }
}
```

Of course a different configuration option could be used to specify the mapping of labels to URLs in a more generic way, reusable by other types of charts.

Start by reviewing the pie-chart dataset configuration and the server-rendered SVG output described in the issue, comparing the proposed URL column with the existing anchor-wrapping approach. Done means pie sections and legend entries can navigate to specified URLs without requiring browser JavaScript, with an API approach that can support other chart types.

Written by the indexing model from the issue text.