group-events.js

const SENTINEL = -Infinity;

function groupEvents(events) {
  if (!Array.isArray(events) || events.length === 0) {
    return [];
  }
  events = events.slice();
  events.sort((a, b) => a.start - b.start);

  let group,
    minColumnEnd = SENTINEL,
    prevEnd = SENTINEL,
    currentColumns = [SENTINEL];

  for (let i = 0, event; event = events[i]; i++) {
    let candColumn = -1, columnClosed = 0;
    if (minColumnEnd < event.start) {
      for (let j = 0, columnEnd; columnEnd = currentColumns[j]; j++) {
        if (columnEnd < event.start) {
          columnEnd = j;
          columnClosed++;
        }
      }
    }
    if (columnClosed === currentColumns.length) {
      group = { columns: 1 };
      currentColumns = [SENTINEL];
      candColumn = 0;
    } else if (candColumn === -1) {
      currentColumns.push(SENTINEL);
      candColumn = currentColumns.length - 1;
      group.columns++;
    }
    currentColumns[candColumn] = event.end;
    minColumnEnd = Math.min(minColumnEnd, currentColumns[candColumn]);
    event.group = group;
    event.column = candColumn;
  }
  return events;
}

// http://jsbin.com/qazixew/edit?js,output