feat: Added Table.plot_boxplots to plot a boxplot for each numerica…

…l column in the table (#254)

Closes #156.

### Summary of Changes

Added `Table.plot_boxplots` to plot a boxplot for each numerical column
in the table
Changed `Column.plot_boxplot` to set the title instead of the xlabel to
match `Table.plot_boxplots`

### Additional Context

Waiting for #239 -> Needed to add tests for `Table.plot_boxplots`

---------

Co-authored-by: sibre28 <[email protected]>
Co-authored-by: alex-senger <[email protected]>
Co-authored-by: megalinter-bot <[email protected]>
Co-authored-by: Lars Reimann <[email protected]>

docs/tutorials/data_visualization.ipynb

@@ -186,6 +186,26 @@
     }
    }
   },
+  {
+   "cell_type": "markdown",
+   "source": [
+    "## Boxplot of all numerical columns"
+   ],
+   "metadata": {
+    "collapsed": false
+   }
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "outputs": [],
+   "source": [
+    "titanic_numerical.plot_boxplots()"
+   ],
+   "metadata": {
+    "collapsed": false
+   }
+  },
   {
    "cell_type": "markdown",
    "source": [

src/safeds/data/tabular/containers/_column.py

@@ -596,7 +596,8 @@ def plot_boxplot(self) -> Image:
 
         fig = plt.figure()
         ax = sns.boxplot(data=self._data)
-        ax.set(xlabel=self.name)
+        ax.set(title=self.name)
+        ax.set_xticks([])
         plt.tight_layout()
 
         buffer = io.BytesIO()

src/safeds/data/tabular/containers/_table.py

@@ -2,6 +2,7 @@
 
 import functools
 import io
+import warnings
 from pathlib import Path
 from typing import TYPE_CHECKING, Any
 
@@ -1313,6 +1314,47 @@ def plot_scatterplot(self, x_column_name: str, y_column_name: str) -> Image:
         buffer.seek(0)
         return Image(buffer, format_=ImageFormat.PNG)
 
+    def plot_boxplots(self) -> Image:
+        """
+        Plot a boxplot for every numerical column.
+
+        Returns
+        -------
+        plot: Image
+            The plot as an image.
+
+        Raises
+        ------
+        NonNumericColumnError
+            If the table contains only non-numerical columns.
+        """
+        numerical_table = self.remove_columns_with_non_numerical_values()
+        if numerical_table.number_of_columns == 0:
+            raise NonNumericColumnError("This table contains only non-numerical columns.")
+        col_wrap = min(numerical_table.number_of_columns, 3)
+
+        data = pd.melt(numerical_table._data, value_vars=numerical_table.column_names)
+        grid = sns.FacetGrid(data, col="variable", col_wrap=col_wrap, sharex=False, sharey=False)
+        with warnings.catch_warnings():
+            warnings.filterwarnings(
+                "ignore",
+                message="Using the boxplot function without specifying `order` is likely to produce an incorrect plot.",
+            )
+            grid.map(sns.boxplot, "variable", "value")
+        grid.set_xlabels("")
+        grid.set_ylabels("")
+        grid.set_titles("{col_name}")
+        for axes in grid.axes.flat:
+            axes.set_xticks([])
+        plt.tight_layout()
+        fig = grid.fig
+
+        buffer = io.BytesIO()
+        fig.savefig(buffer, format="png")
+        plt.close()  # Prevents the figure from being displayed directly
+        buffer.seek(0)
+        return Image(buffer, format_=ImageFormat.PNG)
+
     def plot_histograms(self) -> Image:
         """
         Plot a histogram for every column.

tests/safeds/data/tabular/containers/_column/test_plot_boxplot.py

@@ -8,7 +8,6 @@
 
 def test_should_match_snapshot() -> None:
     table = Table({"A": [1, 2, 3]})
-    table.get_column("A").plot_boxplot()
     current = table.get_column("A").plot_boxplot()
     snapshot = Image.from_png_file(resolve_resource_path("./image/snapshot_boxplot.png"))
 

tests/safeds/data/tabular/containers/_table/test_plot_boxplots.py

@@ -0,0 +1,37 @@
+import pytest
+from safeds.data.image.containers import Image
+from safeds.data.tabular.containers import Table
+from safeds.exceptions import NonNumericColumnError
+
+from tests.helpers import resolve_resource_path
+
+
+@pytest.mark.parametrize(
+    ("table", "path"),
+    [
+        (Table({"A": [1, 2, 3]}), "./image/snapshot_boxplots/one_column.png"),
+        (
+            Table({"A": [1, 2, 3], "B": ["A", "A", "Bla"], "C": [True, True, False], "D": [1.0, 2.1, 4.5]}),
+            "./image/snapshot_boxplots/four_columns_some_non_numeric.png",
+        ),
+        (
+            Table({"A": [1, 2, 3], "B": [1.0, 2.1, 4.5], "C": [1, 2, 3], "D": [1.0, 2.1, 4.5]}),
+            "./image/snapshot_boxplots/four_columns_all_numeric.png",
+        ),
+    ],
+    ids=["one column", "four columns (some non-numeric)", "four columns (all numeric)"],
+)
+def test_should_match_snapshot(table: Table, path: str) -> None:
+    current = table.plot_boxplots()
+    current.to_png_file(resolve_resource_path(path))
+    snapshot = Image.from_png_file(resolve_resource_path(path))
+
+    # Inlining the expression into the assert causes pytest to hang if the assertion fails when run from PyCharm.
+    assertion = snapshot._image.tobytes() == current._image.tobytes()
+    assert assertion
+
+
+def test_should_raise_if_column_contains_non_numerical_values() -> None:
+    table = Table.from_dict({"A": ["1", "2", "3.5"], "B": ["0.2", "4", "77"]})
+    with pytest.raises(NonNumericColumnError):
+        table.plot_boxplots()

tests/safeds/data/tabular/containers/_table/test_plot_histograms.py

@@ -14,6 +14,7 @@
             "./image/snapshot_histograms/four_columns.png",
         ),
     ],
+    ids=["one column", "four columns"],
 )
 def test_should_match_snapshot(table: Table, path: str) -> None:
     current = table.plot_histograms()