diff --git a/src/ert/gui/plottery/plots/histogram.py b/src/ert/gui/plottery/plots/histogram.py
index f7e7ad0634d..51e5be44b12 100644
--- a/src/ert/gui/plottery/plots/histogram.py
+++ b/src/ert/gui/plottery/plots/histogram.py
@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from math import ceil, floor, log10, sqrt
-from typing import TYPE_CHECKING, Any, Dict, List
+from typing import TYPE_CHECKING, Dict, List, Optional
 
 import numpy
 import pandas as pd
@@ -15,7 +15,7 @@
     from matplotlib.axes import Axes
     from matplotlib.figure import Figure
 
-    from ert.gui.plottery import PlotConfig, PlotContext
+    from ert.gui.plottery import PlotContext, PlotStyle
 
 
 class HistogramPlot:
@@ -27,17 +27,17 @@ def plot(
         figure: Figure,
         plot_context: PlotContext,
         ensemble_to_data_map: Dict[EnsembleObject, pd.DataFrame],
-        _observation_data: Any,
-        std_dev_images: Any,
+        observation_data: pd.DataFrame,
+        std_dev_images: Dict[str, bytes],
     ) -> None:
-        plotHistogram(figure, plot_context, ensemble_to_data_map, _observation_data)
+        plotHistogram(figure, plot_context, ensemble_to_data_map, observation_data)
 
 
 def plotHistogram(
     figure: Figure,
     plot_context: PlotContext,
     ensemble_to_data_map: Dict[EnsembleObject, pd.DataFrame],
-    _observation_data: Any,
+    observation_data: pd.DataFrame,
 ) -> None:
     config = plot_context.plotConfig()
 
@@ -58,18 +58,22 @@ def plotHistogram(
     plot_context.x_axis = plot_context.VALUE_AXIS
     plot_context.y_axis = plot_context.COUNT_AXIS
 
-    if config.xLabel() is None:
-        config.setXLabel("Value")
+    x_label = config.xLabel()
+    if x_label is None:
+        x_label = "Value"
+    config.setXLabel(x_label)
 
-    if config.yLabel() is None:
-        config.setYLabel("Count")
+    y_label = config.yLabel()
+    if y_label is None:
+        y_label = "Count"
+    config.setYLabel(y_label)
 
     use_log_scale = plot_context.log_scale
 
     data = {}
     minimum = None
     maximum = None
-    categories = set()
+    categories: set[str] = set()
     max_element_count = 0
     categorical = False
     for ensemble, datas in ensemble_to_data_map.items():
@@ -101,7 +105,6 @@ def plotHistogram(
             maximum = current_max if maximum is None else max(maximum, current_max)
             max_element_count = max(max_element_count, len(data[ensemble.name].index))
 
-    categories = sorted(categories)
     bin_count = int(ceil(sqrt(max_element_count)))
 
     axes = {}
@@ -111,29 +114,35 @@ def plotHistogram(
         axes[ensemble.name].set_title(
             f"{config.title()} ({ensemble.experiment_name} : {ensemble.name})"
         )
+        axes[ensemble.name].set_xlabel(x_label)
+        axes[ensemble.name].set_ylabel(y_label)
 
         if use_log_scale:
             axes[ensemble.name].set_xscale("log")
 
         if not data[ensemble.name].empty:
             if categorical:
-                _plotCategoricalHistogram(
-                    axes[ensemble.name],
-                    config,
-                    data[ensemble.name],
+                config.addLegendItem(
                     ensemble.name,
-                    categories,
+                    _plotCategoricalHistogram(
+                        axes[ensemble.name],
+                        config.histogramStyle(),
+                        data[ensemble.name],
+                        sorted(categories),
+                    ),
                 )
             else:
-                _plotHistogram(
-                    axes[ensemble.name],
-                    config,
-                    data[ensemble.name],
+                config.addLegendItem(
                     ensemble.name,
-                    bin_count,
-                    use_log_scale,
-                    minimum,
-                    maximum,
+                    _plotHistogram(
+                        axes[ensemble.name],
+                        config.histogramStyle(),
+                        data[ensemble.name],
+                        bin_count,
+                        use_log_scale,
+                        minimum,
+                        maximum,
+                    ),
                 )
 
             config.nextColor()
@@ -157,16 +166,10 @@ def plotHistogram(
 
 def _plotCategoricalHistogram(
     axes: "Axes",
-    plot_config: "PlotConfig",
+    style: PlotStyle,
     data: pd.DataFrame,
-    label: str,
     categories: List[str],
-):
-    axes.set_xlabel(plot_config.xLabel())
-    axes.set_ylabel(plot_config.yLabel())
-
-    style = plot_config.histogramStyle()
-
+) -> Rectangle:
     counts = data.value_counts()
     freq = [counts.get(category, 0) for category in categories]
     pos = numpy.arange(len(categories))
@@ -176,27 +179,20 @@ def _plotCategoricalHistogram(
 
     axes.bar(pos, freq, alpha=style.alpha, color=style.color, width=width)
 
-    rectangle = Rectangle(
+    return Rectangle(
         (0, 0), 1, 1, color=style.color
     )  # creates rectangle patch for legend use.
-    plot_config.addLegendItem(label, rectangle)
 
 
 def _plotHistogram(
     axes: "Axes",
-    plot_config: "PlotConfig",
+    style: PlotStyle,
     data: pd.DataFrame,
-    label: str,
-    bin_count,
-    use_log_scale=False,
-    minimum=None,
-    maximum=None,
-):
-    axes.set_xlabel(plot_config.xLabel())
-    axes.set_ylabel(plot_config.yLabel())
-
-    style = plot_config.histogramStyle()
-
+    bin_count: int,
+    use_log_scale: float = False,
+    minimum: Optional[float] = None,
+    maximum: Optional[float] = None,
+) -> Rectangle:
     if minimum is not None and maximum is not None:
         if use_log_scale:
             bins = _histogramLogBins(bin_count, minimum, maximum)
@@ -213,10 +209,9 @@ def _plotHistogram(
 
     axes.set_xlim(minimum, maximum)
 
-    rectangle = Rectangle(
+    return Rectangle(
         (0, 0), 1, 1, color=style.color
     )  # creates rectangle patch for legend use.'
-    plot_config.addLegendItem(label, rectangle)
 
 
 def _histogramLogBins(bin_count: int, minimum: float, maximum: float):