add test and docstring to the density method

Author: MAfarrag

docs/source/_images/time_series/density-1d.png

@@ -777,15 +777,29 @@ def density(self, **kwargs) -> Tuple[Figure, Axes]:
 
         Examples
         --------
-        >>> ts = TimeSeries(np.random.randn(100))
-        >>> fig, ax = ts.plot_density()
+        - Plot the KDE density plot for a 1D time series:
+
+            >>> ts = TimeSeries(np.random.randn(100))
+            >>> fig, ax = ts.density(title='Density Plot', xlabel='Random Values', ylabel='KDE density')
+
+            .. image:: /_images/time_series/density-1d.png
+                    :align: center
+
+        - Plot the KDE density plot for a 2D time series:
+
+            >>> ts = TimeSeries(np.random.randn(100, 4))
+            >>> fig, ax = ts.density(title='Density Plot', xlabel='Random Values', ylabel='KDE density')
+
+            .. image:: /_images/time_series/density-2d.png
+                    :align: center
         """
         fig, ax = self._get_ax_fig(**kwargs)
         color = kwargs.get("color", None)
-        self[self.columns].plot(kind="density", ax=ax, color=color)
+        self[self.columns.to_list()].plot(kind="density", ax=ax, color=color)
+        kwargs.pop("ax", None)
         ax = self._adjust_axes_labels(
             ax,
-            self.columns,
+            kwargs.get("xtick_labels"),
             **kwargs,
         )
 

docs/source/_images/time_series/density-2d.png

@@ -226,3 +226,21 @@ def test_plot_rolling_statistics(self, ts: TimeSeries, request):
         fig2, ax2 = ts.rolling_statistics(fig=fig, ax=ax)
         assert fig2 is fig, "If fig is provided, plot_rolling_statistics should use it."
         assert ax2 is ax, "If ax is provided, plot_rolling_statistics should use it."
+
+
+class TestDensity:
+
+    @pytest.mark.parametrize("ts", ["ts_1d", "ts_2d"])
+    def test_plot_density(self, ts: TimeSeries, request):
+        """Test the plot_density method."""
+        ts = request.getfixturevalue(ts)
+        fig, ax = ts.density()
+        assert isinstance(
+            fig, plt.Figure
+        ), "plot_density should return a matplotlib Figure."
+        assert isinstance(ax, plt.Axes), "plot_density should return a matplotlib Axes."
+
+        fig, ax = plt.subplots()
+        fig2, ax2 = ts.density(fig=fig, ax=ax)
+        assert fig2 is fig, "If fig is provided, plot_density should use it."
+        assert ax2 is ax, "If ax is provided, plot_density should use it."