Binding selection param to axis for interaction

[tonyxiao]

It is already possible to bind selection to a legend (https://vega.github.io/vega-lite/docs/bind.html)

However sometimes it is a lot more convenient to be interaction with axis label instead, especially when the data point represented is so small that it is very difficult to select it otherwise. Here's an illustration of the problem.

Here's what I'm thinking of

"params": [{
    "name": "currency",
    "select": {"type": "point"},
    "bind": "axisLabel"
  }]

[domoritz]

Duplicate of #1657. Please add your example to the other thread.