Makes heatmaps "clickable"

Nobody has claimed this yet.

Hello!
I would like to have every single square clickable. The use case I have in my mind is to jump to a different visualization of the particular event from the heatmap.

Let's say that I am visualizing the HTTP request latency and every square represent a request, I would like to be able to go to my trace, or to the dashboard that visualizes a single request clicking the square.

In Chronograf we can have a convention: "if the point returns a `_link` field it is the target of the square" in this way we can use a query to compose and inject the `_link` to the graph.

The issue does not name any files, tests, or entry points. First clarify how heatmap squares should receive and follow a `_link` value, and whether the behavior belongs in Giraffe or Chronograf; done should include an agreed interaction and tests for clickable squares.

Written by the indexing model from the issue text.