d3article4.2

const nestByDate = d3.nest()
  .key(d => d.date)
  .entries(data);

const percentsByDate = {};

nestByDate.forEach(dateItem => {
  percentsByDate[dateItem.key] = {};

  dateItem.values.forEach(item => {
    percentsByDate[dateItem.key][item.regionId] = item.percent;
  });
});