Fix transect title and units

The data array attributes were being dropped in
`Transect.prepare_data_array_for_transect`.

A new `emsarray.plot.make_plot_title()` function has been added.
It is used for surface plots and transects.

Author: mx-moth

src/emsarray/conventions/_base.py

@@ -23,7 +23,8 @@
 from emsarray.exceptions import NoSuchCoordinateError
 from emsarray.operations import depth
 from emsarray.plot import (
-    _requires_plot, animate_on_figure, plot_on_figure, polygons_to_collection
+    _requires_plot, animate_on_figure, make_plot_title, plot_on_figure,
+    polygons_to_collection
 )
 from emsarray.state import State
 from emsarray.types import Bounds, Pathish
@@ -938,25 +939,7 @@ def plot_on_figure(
             #
             # Users can supply their own titles
             # if this automatic behaviour is insufficient
-            title_bits: list[str] = []
-            long_name = kwargs['scalar'].attrs.get('long_name')
-            if long_name is not None:
-                title_bits.append(str(long_name))
-            try:
-                time_coordinate = self.dataset.variables[self.get_time_name()]
-            except KeyError:
-                pass
-            else:
-                # Add a time stamp when the time coordinate has a single value.
-                # This happens when you `.sel()` a single time slice to plot -
-                # as long as the time coordinate is a proper coordinate with
-                # matching dimension name, not an auxiliary coordinate.
-                if time_coordinate.size == 1:
-                    time = time_coordinate.values[0]
-                    title_bits.append(str(time))
-
-            if title_bits:
-                kwargs['title'] = '\n'.join(title_bits)
+            kwargs['title'] = make_plot_title(self.dataset, kwargs['scalar'])
 
         plot_on_figure(figure, self, **kwargs)
 

src/emsarray/plot.py

@@ -8,6 +8,7 @@
 import numpy
 import xarray
 
+from emsarray.exceptions import NoSuchCoordinateError
 from emsarray.types import Landmark
 from emsarray.utils import requires_extra
 
@@ -217,6 +218,48 @@ def polygons_to_collection(
     )
 
 
+def make_plot_title(
+    dataset: xarray.Dataset,
+    data_array: xarray.DataArray,
+) -> Optional[str]:
+    """
+    Make a suitable plot title for a variable.
+    This will attempt to find a name for the variable by looking through the attributes.
+    If the variable has a time coordinate,
+    and the time coordinate has a single value,
+    the time step is appended after the title.
+    """
+    if 'long_name' in data_array.attrs:
+        title = str(data_array.attrs['long_name'])
+    elif 'standard_name' in data_array.attrs:
+        title = str(data_array.attrs['standard_name'])
+    elif data_array.name is not None:
+        title = str(data_array.name)
+    else:
+        return None
+
+    # Check if this variable has a time coordinate
+    try:
+        time_coordinate = dataset.ems.time_coordinate
+    except NoSuchCoordinateError:
+        return title
+    if time_coordinate.name not in data_array.coords:
+        return title
+    # Fetch the coordinate from the data array itself,
+    # in case someone did `data_array = dataset['temp'].isel(time=0)`
+    time_coordinate = data_array.coords[time_coordinate.name]
+
+    if len(time_coordinate.dims) == 0:
+        time_value = time_coordinate.values
+    elif time_coordinate.size == 1:
+        time_value = time_coordinate.values[0]
+    else:
+        return title
+
+    time_string = numpy.datetime_as_string(time_value, unit='auto')
+    return f'{title}\n{time_string}'
+
+
 @_requires_plot
 def plot_on_figure(
     figure: Figure,

src/emsarray/transect.py

@@ -20,7 +20,7 @@
 from matplotlib.ticker import EngFormatter, Formatter
 
 from emsarray.conventions import Convention, Index
-from emsarray.plot import _requires_plot
+from emsarray.plot import _requires_plot, make_plot_title
 from emsarray.types import Landmark
 from emsarray.utils import move_dimensions_to_end
 
@@ -493,6 +493,10 @@ def prepare_data_array_for_transect(self, data_array: xarray.DataArray) -> xarra
             The input data array transformed to have the correct shape
             for plotting on the transect.
         """
+        # Some of the following operations drop attrs,
+        # so keep a reference to the original ones
+        attrs = data_array.attrs
+
         data_array = self.convention.ravel(data_array)
 
         depth_dimension = self.transect_dataset.coords['depth'].dims[0]
@@ -502,6 +506,9 @@ def prepare_data_array_for_transect(self, data_array: xarray.DataArray) -> xarra
         linear_indices = self.transect_dataset['linear_index'].values
         data_array = data_array.isel({index_dimension: linear_indices})
 
+        # Restore attrs after reformatting
+        data_array.attrs.update(attrs)
+
         return data_array
 
     def _find_depth_bounds(self, data_array: xarray.DataArray) -> Tuple[int, int]:
@@ -749,7 +756,7 @@ def _plot_on_figure(
         axes.set_ylim(depth_limit_deep, depth_limit_shallow)
 
         if title is None:
-            title = data_array.attrs.get('long_name')
+            title = make_plot_title(self.dataset, data_array)
         if title is not None:
             axes.set_title(title)
 

tests/conventions/test_base.py

@@ -443,6 +443,7 @@ def test_plot():
         'botz': (['y', 'x'], numpy.random.standard_normal((10, 20)) - 10),
     })
     convention = SimpleConvention(dataset)
+    convention.bind()
 
     # Naming a simple variable should work fine
     convention.plot('botz')

tests/test_plot.py

@@ -58,19 +58,15 @@ def test_plot(
     datasets: pathlib.Path,
     tmp_path: pathlib.Path,
 ):
-    """
-    Test plotting a variable with no long_name attribute works.
-    Regression test for https://github.com/csiro-coasts/emsarray/issues/105
-    """
-    dataset = emsarray.tutorial.open_dataset('gbr4')
+    dataset = emsarray.tutorial.open_dataset('fraser')
     temp = dataset['temp'].copy()
     temp = temp.isel(time=0, k=-1)
 
     dataset.ems.plot(temp)
 
     figure = matplotlib.pyplot.gcf()
     axes = figure.axes[0]
-    assert axes.get_title() == 'Temperature\n2022-05-11T14:00:00.000000000'
+    assert axes.get_title() == 'Temperature\n2022-05-11T14:00'
 
     matplotlib.pyplot.savefig(tmp_path / 'plot.png')
     logger.info("Saved plot to %r", tmp_path / 'plot.png')
@@ -86,7 +82,7 @@ def test_plot_no_long_name(
     Test plotting a variable with no long_name attribute works.
     Regression test for https://github.com/csiro-coasts/emsarray/issues/105
     """
-    dataset = emsarray.tutorial.open_dataset('gbr4')
+    dataset = emsarray.tutorial.open_dataset('fraser')
     temp = dataset['temp'].copy()
     temp = temp.isel(time=0, k=-1)
     del temp.attrs['long_name']
@@ -95,7 +91,7 @@ def test_plot_no_long_name(
 
     figure = matplotlib.pyplot.gcf()
     axes = figure.axes[0]
-    assert axes.get_title() == '2022-05-11T14:00:00.000000000'
+    assert axes.get_title() == 'temp\n2022-05-11T14:00'
 
     matplotlib.pyplot.savefig(tmp_path / 'plot.png')
     logger.info("Saved plot to %r", tmp_path / 'plot.png')

tests/test_transect.py

@@ -0,0 +1,57 @@
+import logging
+import pathlib
+
+import matplotlib
+import pytest
+import shapely
+
+import emsarray.transect
+
+logger = logging.getLogger(__name__)
+
+
+@pytest.mark.matplotlib(mock_coast=True)
+@pytest.mark.tutorial
+def test_plot(
+    datasets: pathlib.Path,
+    tmp_path: pathlib.Path,
+):
+    dataset = emsarray.tutorial.open_dataset('gbr4')
+    temp = dataset['temp'].copy()
+    temp = temp.isel(time=-1)
+
+    line = shapely.LineString([
+        [152.9768944, -25.4827962],
+        [152.9701996, -25.4420345],
+        [152.9727745, -25.3967620],
+        [152.9623032, -25.3517828],
+        [152.9401588, -25.3103560],
+        [152.9173279, -25.2538563],
+        [152.8962135, -25.1942238],
+        [152.8692627, -25.0706729],
+        [152.8623962, -24.9698750],
+        [152.8472900, -24.8415806],
+        [152.8308105, -24.6470172],
+        [152.7607727, -24.3521012],
+        [152.6392365, -24.1906056],
+        [152.4792480, -24.0615124],
+    ])
+    emsarray.transect.plot(
+        dataset, line, temp,
+        bathymetry=dataset['botz'])
+
+    figure = matplotlib.pyplot.gcf()
+    axes = figure.axes[0]
+    # This is assembled from the variable long_name and the time coordinate
+    assert axes.get_title() == 'Temperature\n2022-05-11T14:00'
+    # This is the long_name of the depth coordinate
+    assert axes.get_ylabel() == 'Z coordinate'
+    # This is made up
+    assert axes.get_xlabel() == 'Distance along transect'
+
+    colorbar = figure.axes[-1]
+    # This is the variable units
+    assert colorbar.get_ylabel() == 'degrees C'
+
+    matplotlib.pyplot.savefig(tmp_path / 'plot.png')
+    logger.info("Saved plot to %r", tmp_path / 'plot.png')