Add a frequency table of events at each reference line.

[samussiah]

Addressed in reference_line_tables.

Test notes

• For each reference line a corresponding frequency table appears in the left column beneath the controls.
• The title of the frequency table matches the reference line tooltip.
• The frequency table breaks down the events that overlap the reference line by event type, and if the timelines are grouped, by the group variable.

Use settings below:

{
  "groupings":[
    {"value_col":"SEX","label":"sex"}
  ],
  "reference_lines":[
    {"timepoint":"2015-05-01","label":"timepoint"}
  ]
}

[dschwentker]

This has been tested and passed in all browsers.