function createColumns(group) {
  const columns = []
  const eventStackMap = {}

  group.forEach(event => {
    let column = eventStackMap[event.id]
    if (!column) {
      column = eventStackMap[event.id]
      column = [event]
      columns.push(column)
    }

    group.forEach(_event => {
      if (_event === event) return
      if (!collide(event, _event)) {
        if (!eventStackMap[_event.id]) {
          eventStackMap[_event.id] = column
          column.push(_event)
        }
      }
    })
  })

  return columns
}
Create columns within a group. To avoid visual overlap.

@param {Array} events
@return {Array}