fix: flow bins xticklabel correction (#502)

* Fixed index issue in drawing flow bin xticks and added support for drawing flow bins on shared x axes

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Initialize xticks, xticklabels as empty array and list instead of None

* Filter for shared axes in same column

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Removed drawing dashed line on top of axis

* Modified xticklabel formatting to truncate trailing zeros and updated reference images.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/mplhep/plot.py

@@ -550,68 +550,125 @@ def iterable_not_string(arg):
                 transform=ax.get_xaxis_transform(),
             )
 
-    elif flow == "show" and (underflow > 0.0 or overflow > 0.0):
-        xticks = [label.get_text() for label in ax.get_xticklabels()]
+    elif flow == "show":
+        underflow_xticklabel = f"<{flow_bins[1]:.2g}"
+        overflow_xticklabel = f">{flow_bins[-2]:.2g}"
+
+        # Loop over shared x axes to get xticks and xticklabels
+        xticks, xticklabels = np.array([]), []
+        shared_axes = ax.get_shared_x_axes().get_siblings(ax)
+        shared_axes = [
+            _ax for _ax in shared_axes if _ax.get_position().x0 == ax.get_position().x0
+        ]
+        for _ax in shared_axes:
+            _xticks = _ax.get_xticks()
+            _xticklabels = [label.get_text() for label in _ax.get_xticklabels()]
+
+            # Check if underflow/overflow xtick already exists
+            if (
+                underflow_xticklabel in _xticklabels
+                or overflow_xticklabel in _xticklabels
+            ):
+                xticks = _xticks
+                xticklabels = _xticklabels
+                break
+            elif len(_xticklabels) > 0:
+                xticks = _xticks
+                xticklabels = _xticklabels
+
         lw = ax.spines["bottom"].get_linewidth()
         _edges = plottables[0].edges
         _centers = plottables[0].centers
         _marker_size = (
             20
             * ax.get_window_extent().transformed(fig.dpi_scale_trans.inverted()).width
         )
-        if underflow > 0.0:
-            xticks[0] = ""
-            xticks[1] = f"<{flow_bins[2]}"
-            ax.set_xticks(ax.get_xticks())
-            ax.set_xticklabels(xticks)
 
-            ax.plot(
-                [_edges[0], _edges[1]],
-                [0, 0],
-                color="white",
-                zorder=5,
-                ls="--",
-                lw=lw,
-                transform=ax.get_xaxis_transform(),
-                clip_on=False,
-            )
-            ax.scatter(
-                _centers[0],
-                0,
-                _marker_size,
-                marker=align_marker("d", valign="center"),
-                edgecolor="black",
-                zorder=5,
-                clip_on=False,
-                facecolor="white",
-                transform=ax.get_xaxis_transform(),
-            )
-        if overflow > 0.0:
-            xticks[-1] = ""
-            xticks[-2] = f">{flow_bins[-3]}"
-            ax.set_xticks(ax.get_xticks())
-            ax.set_xticklabels(xticks)
-            ax.plot(
-                [_edges[-2], _edges[-1]],
-                [0, 0],
-                color="white",
-                zorder=5,
-                ls="--",
-                lw=lw,
-                transform=ax.get_xaxis_transform(),
-                clip_on=False,
-            )
-            ax.scatter(
-                _centers[-1],
-                0,
-                _marker_size,
-                marker=align_marker("d", valign="center"),
-                edgecolor="black",
-                zorder=5,
-                clip_on=False,
-                facecolor="white",
-                transform=ax.get_xaxis_transform(),
-            )
+        if underflow > 0.0 or underflow_xticklabel in xticklabels:
+            # Replace any existing xticks in underflow region with underflow bin center
+            _mask = xticks > flow_bins[1]
+            xticks = np.insert(xticks[_mask], 0, _centers[0])
+            xticklabels = [underflow_xticklabel] + [
+                xlab for i, xlab in enumerate(xticklabels) if _mask[i]
+            ]
+
+            # Don't draw markers on the top of the top axis
+            top_axis = max(shared_axes, key=lambda a: a.get_position().y0)
+
+            # Draw on all shared axes
+            for _ax in shared_axes:
+                _ax.set_xticks(xticks)
+                _ax.set_xticklabels(xticklabels)
+                for h in [0, 1]:
+                    # Don't draw marker on the top of the top axis
+                    if _ax == top_axis and h == 1:
+                        continue
+
+                    _ax.plot(
+                        [_edges[0], _edges[1]],
+                        [h, h],
+                        color="white",
+                        zorder=5,
+                        ls="--",
+                        lw=lw,
+                        transform=_ax.get_xaxis_transform(),
+                        clip_on=False,
+                    )
+
+                    _ax.scatter(
+                        _centers[0],
+                        h,
+                        _marker_size,
+                        marker=align_marker("d", valign="center"),
+                        edgecolor="black",
+                        zorder=5,
+                        clip_on=False,
+                        facecolor="white",
+                        transform=_ax.get_xaxis_transform(),
+                    )
+        if overflow > 0.0 or overflow_xticklabel in xticklabels:
+            # Replace any existing xticks in overflow region with overflow bin center
+            _mask = xticks < flow_bins[-2]
+            xticks = np.insert(xticks[_mask], sum(_mask), _centers[-1])
+            xticklabels = [xlab for i, xlab in enumerate(xticklabels) if _mask[i]] + [
+                overflow_xticklabel
+            ]
+
+            # Don't draw markers on the top of the top axis
+            top_axis = max(shared_axes, key=lambda a: a.get_position().y0)
+
+            # Draw on all shared axes
+            for _ax in shared_axes:
+                _ax.set_xticks(xticks)
+                _ax.set_xticklabels(xticklabels)
+
+                for h in [0, 1]:
+                    # Don't draw marker on the top of the top axis
+                    if _ax == top_axis and h == 1:
+                        continue
+
+                    _ax.plot(
+                        [_edges[-2], _edges[-1]],
+                        [h, h],
+                        color="white",
+                        zorder=5,
+                        ls="--",
+                        lw=lw,
+                        transform=_ax.get_xaxis_transform(),
+                        clip_on=False,
+                    )
+
+                    _ax.scatter(
+                        _centers[-1],
+                        h,
+                        _marker_size,
+                        marker=align_marker("d", valign="center"),
+                        edgecolor="black",
+                        zorder=5,
+                        clip_on=False,
+                        facecolor="white",
+                        transform=_ax.get_xaxis_transform(),
+                    )
 
     return return_artists