x-axis order not preserved in split series vislib charts

[lukeelmers]

Currently the x-axis sort order is not always respected when working with vislib charts that have a split series.

To Reproduce:

1. Create a stacked bar chart using kibana_sample_data_logs
2. Set time range to today
3. Metrics:
   • Y-Axis: Count
4. Buckets:
   • X-Axis: terms on extension.keyword, order: alphabetical ascending, size: 5
   • Split Series: terms on machine.os.keyword, order: count descending, size: 3
5. The values along the x-axis are not in alphabetical order.

This is due to the way the point series agg response handler generates series data, only filling in series values as it encounters them bucket-by-bucket. By the time the series data makes it to vislib, the original sort order from ES & the tabified data is lost.

This was discovered as a part of the investigation into #17532.

[elasticmachine]

Pinging @elastic/kibana-app