[BUG] xAxis: Date mismatch in combined charts

[Daveiano]

A mismatching of the date on the xAxis can be noticed when using the cumulative aggregation:

To investigate

• Could be something with start vs end timestamp (https://github.com/Daveiano/weewx-wdc/blob/3.x/skins/weewx-wdc/src/js/index.tsx#L133)

return {
  x: item[0],
  y: item[2],
  start: item[0],
  end: item[1],
};

• Could be something with the xAxis scale (https://github.com/Daveiano/weewx-wdc/blob/3.x/skins/weewx-wdc/src/js/diagrams/d3/combined.tsx#L272)