How can display horizontal bar on the Timelion? (or another chart)

Elastic Stack Kibana
4

Have a look at this answer here with a semi-full Vega configuration: Gantt chart on Kibana using version 7.11 - #4 by Marco_Liberati

In the y encoding you might use always the same value (so either a field with a static value, or a static string) to have a single "swimlane":

encoding: {
    y: {
      field: _source.Date // <== change this in the linked configuration
      type: nominal
    }
...