[BUG] Heatmap Y-axis does not sort correctly

[ghost]

Describe the bug

The Y-axis of a heatmap visualization does not sort correctly when using (at least) a Histogram or Date Histogram aggregation for that axis.

To Reproduce
Steps to reproduce the behavior:

1. Create a heatmap visualization
2. Use a date histogram for the X-axis
3. Use either a date histogram or (regular) histogram for the y-axis

Expected behavior
Y-axis buckets should be sorted

OpenSearch Version
"OpenSearch 1.0" (via AWS OpenSearch)

Dashboards Version
OpenSearch 1.0

Plugins

N/A

Screenshots

Host/Environment (please complete the following information):

• OS: macOS 10.15.7
• Chrome 94.0.4606.61 (Official Build) (x86_64)

Additional context

HeatMap is the closest thing OpenSearch / Kibana offers to a scatterplot, and has historically worked ok for this use case. However this bug severely limits eliminates heatmap's usefulness for visualizing two dimensional data.

[adrien-cahoreau]

The sort bug on sub-aggregations affects not only the heatmap, but also other visualisations.
Since years, there are several reports of this bug on Kibana :

• Sort sub-aggregation axis by term not working elastic/kibana#10648
• Sub-aggregations should be sorted globally elastic/kibana#14536
• Bar in a bar charts are getting unsorted when adding a sub-bucket aggregation elastic/kibana#17532

And Elastic plans to fix it mainly in its own Lens : elastic/kibana#86184

However, this bug has been fixed recently on the Kibana's heatmap (8.1) with a new implementation from elastic-charts : elastic/kibana#118338

[WhereDidTheBirdsGo]

As a workaround, try setting the 'extend bounds' values for the Y to encompassing values. Not perfect, for me it sorted it upside-down, but at least it's readable now.