Add histogram and spaghetti plot visualizations to plot_xY

- Add histogram visualization (2D heatmap) with global y-bins
- Add spaghetti plot visualization with configurable num_samples
- Add ETI (Equal-Tailed Interval) support in addition to HDI
- Add comprehensive test suite for all visualization types
- Maintain backward compatibility with hdi_prob parameter
- Update BaseExperiment.plot() to pass through new parameters

Addresses #671

Author: AllenDowney

causalpy/experiments/base.py

@@ -234,7 +234,7 @@ def plot(
         kind : {"ribbon", "histogram", "spaghetti"}, optional
             Type of visualization. Default is "ribbon".
         ci_kind : {"hdi", "eti"}, optional
-            Type of interval for ribbon plots. Default is "hdi" (matching current behavior).
+            Type of interval for ribbon plots. Default is "hdi".
         ci_prob : float, optional
             The size of the credible interval. Default is :data:`~causalpy.constants.HDI_PROB`.
         num_samples : int, optional

causalpy/plot_utils.py

@@ -62,7 +62,7 @@ def plot_xY(
     kind : {"ribbon", "histogram", "spaghetti"}, optional
         Type of visualization. Default is "ribbon".
     ci_kind : {"hdi", "eti"}, optional
-        Type of interval for ribbon plots. Default is "hdi" (matching current behavior).
+        Type of interval for ribbon plots. Default is "hdi".
     num_samples : int, optional
         Number of posterior samples to plot for spaghetti visualization.
         Default is 50.
@@ -213,7 +213,12 @@ def _plot_histogram(
     plot_hdi_kwargs: dict[str, Any] | None,
     label: str | None,
 ) -> tuple[list[Line2D], None]:
-    """2D heatmap of marginal histograms over time (posterior density vs. time)."""
+    """Plot histogram visualization of the posterior as a 2D heatmap.
+
+    Columns are time points (x), rows are y-value bins; cell values are
+    per-time histogram counts, column-normalized for display. The posterior
+    mean line is overlaid on top.
+    """
     if plot_hdi_kwargs is None:
         plot_hdi_kwargs = {}