Fix AlignmentError in to_xarray_dataset when data variables have different setpoints

[Copilot]

DataSet.to_xarray_dataset() raises an AlignmentError when two data variables share a setpoint coordinate but one of them has additional non-grid setpoints (triggering the multi-index export path).

Root cause: Per-parameter sub-datasets are built independently. A non-grid array parameter (e.g. spectrum with setpoints (f_stop, freq) where freq values differ per sweep point) gets exported via xr.Coordinates.from_pandas_multiindex, making f_stop a component of a MultiIndex rather than a standalone dimension. A co-registered scalar parameter with only (f_stop,) gets a plain PandasIndex for f_stop. xr.merge cannot align these incompatible Index objects.

meas.register_parameter(a.spectrum, setpoints=(a.f_stop,))  # array, non-grid → multi_index path
meas.register_parameter(a.bar, setpoints=(a.f_stop,))       # scalar → standalone f_stop dim

dataset.to_xarray_dataset()
# AlignmentError: cannot align objects on coordinate 'foobar_f_stop'
# because of conflicting indexes

Fix (export_to_xarray.py):

• Before xr.merge, detect sub-datasets that use a multi_index dimension whose coordinate levels conflict with standalone dimensions in other sub-datasets
• Resolve by calling ds.unstack("multi_index") on the conflicting sub-datasets, converting to regular independent dimensions (NaN-filling off-grid entries) so all sub-datasets share a consistent f_stop coordinate structure

Test (test_dataset_export.py):

• Regression test sweeping f_stop over two values with differing frequency grids, verifying both spectrum (2-D, NaN-padded) and bar (1-D) are correctly present with values preserved in the merged dataset

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.11%. Comparing base (355ba4b) to head (4d59746).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8244      +/-   ##
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+ Coverage   71.09%   71.11%   +0.01%     
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  Files         305      305              
  Lines       31934    31947      +13     
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+ Hits        22705    22718      +13     
  Misses       9229     9229              

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[jenshnielsen]

@copilot Please look into the comment on this pr

[Copilot]

Pull request overview

Fixes an AlignmentError in DataSet.to_xarray_dataset() caused by merging per-parameter sub-datasets that represent a shared setpoint coordinate differently (standalone index vs MultiIndex component), by normalizing conflicting MultiIndex sub-datasets before xr.merge.

Changes:

• Detect merge conflicts between MultiIndex level coordinates and standalone dimensions and resolve them by unstack("multi_index") prior to xr.merge.
• Add a regression test covering mixed setpoint structures (non-grid array param + scalar param sharing one setpoint).
• Add a newsfragment documenting the export fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/qcodes/dataset/exporters/export_to_xarray.py	Unstacks conflicting multi_index sub-datasets to avoid xr.merge alignment failures.
tests/dataset/test_dataset_export.py	Adds regression coverage ensuring export succeeds and preserves values with NaN padding for off-grid entries.
docs/changes/newsfragments/8232.improved	Documents the fixed AlignmentError scenario in release notes.