Only use necessary dims when creating temporary dataarray (#9206)

* Only use necessary dims when creating temporary dataarray

* Update dataset_plot.py

* Can't check only data_vars all corrds are no longer added by default

* Update dataset_plot.py

* Add tests

* Update whats-new.rst

* Update dataset_plot.py

doc/whats-new.rst

@@ -37,6 +37,8 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
+- Fix scatter plot broadcasting unneccesarily. (:issue:`9129`, :pull:`9206`)
+  By `Jimmy Westling <https://github.com/illviljan>`_.
 - Don't convert custom indexes to ``pandas`` indexes when computing a diff (:pull:`9157`)
   By `Justus Magin <https://github.com/keewis>`_.
 - Make :py:func:`testing.assert_allclose` work with numpy 2.0 (:issue:`9165`, :pull:`9166`).

xarray/plot/dataset_plot.py

@@ -721,8 +721,8 @@ def _temp_dataarray(ds: Dataset, y: Hashable, locals_: dict[str, Any]) -> DataAr
     """Create a temporary datarray with extra coords."""
     from xarray.core.dataarray import DataArray
 
-    # Base coords:
-    coords = dict(ds.coords)
+    coords = dict(ds[y].coords)
+    dims = set(ds[y].dims)
 
     # Add extra coords to the DataArray from valid kwargs, if using all
     # kwargs there is a risk that we add unnecessary dataarrays as
@@ -732,12 +732,17 @@ def _temp_dataarray(ds: Dataset, y: Hashable, locals_: dict[str, Any]) -> DataAr
     coord_kwargs = locals_.keys() & valid_coord_kwargs
     for k in coord_kwargs:
         key = locals_[k]
-        if ds.data_vars.get(key) is not None:
-            coords[key] = ds[key]
+        darray = ds.get(key)
+        if darray is not None:
+            coords[key] = darray
+            dims.update(darray.dims)
+
+    # Trim dataset from unneccessary dims:
+    ds_trimmed = ds.drop_dims(ds.sizes.keys() - dims)  # TODO: Use ds.dims in the future
 
     # The dataarray has to include all the dims. Broadcast to that shape
     # and add the additional coords:
-    _y = ds[y].broadcast_like(ds)
+    _y = ds[y].broadcast_like(ds_trimmed)
 
     return DataArray(_y, coords=coords)
 

xarray/tests/test_plot.py

@@ -3416,3 +3416,43 @@ def test_9155() -> None:
         data = xr.DataArray([1, 2, 3], dims=["x"])
         fig, ax = plt.subplots(ncols=1, nrows=1)
         data.plot(ax=ax)
+
+
+@requires_matplotlib
+def test_temp_dataarray() -> None:
+    from xarray.plot.dataset_plot import _temp_dataarray
+
+    x = np.arange(1, 4)
+    y = np.arange(4, 6)
+    var1 = np.arange(x.size * y.size).reshape((x.size, y.size))
+    var2 = np.arange(x.size * y.size).reshape((x.size, y.size))
+    ds = xr.Dataset(
+        {
+            "var1": (["x", "y"], var1),
+            "var2": (["x", "y"], 2 * var2),
+            "var3": (["x"], 3 * x),
+        },
+        coords={
+            "x": x,
+            "y": y,
+            "model": np.arange(7),
+        },
+    )
+
+    # No broadcasting:
+    y_ = "var1"
+    locals_ = {"x": "var2"}
+    da = _temp_dataarray(ds, y_, locals_)
+    assert da.shape == (3, 2)
+
+    # Broadcast from 1 to 2dim:
+    y_ = "var3"
+    locals_ = {"x": "var1"}
+    da = _temp_dataarray(ds, y_, locals_)
+    assert da.shape == (3, 2)
+
+    # Ignore non-valid coord kwargs:
+    y_ = "var3"
+    locals_ = dict(x="x", extend="var2")
+    da = _temp_dataarray(ds, y_, locals_)
+    assert da.shape == (3,)