fix(charts): histogram emits PowerPoint's auto-binning form (no binCount child) — issue #14 Phase C

Pareto fixed (b3a2207) but histogram still triggered repair. Precise
byte-diff vs the PowerPoint-authored histogram: the sole histogram-specific
delta was our <cx:binCount>N</cx:binCount> child. dml-chartex.xsd models
binCount/binSize as child elements (our emission was schema-valid) but
PowerPoint's reader rejects that form; PowerPoint's own histogram uses
automatic binning: bare <cx:binning intervalClosed="r"/>.

No PowerPoint ground-truth exists for an explicit-bin form, so per the
no-inference discipline (memory #25: only ground-truth-verified structure
ships) histogram now emits exactly PowerPoint's accepted auto-binning
structure. HistogramChartData still accepts bin_count/bin_size for API
stability but they no longer emit the rejected child — PowerPoint computes
bins from the data (its own default). add_histogram_series signature
simplified; slide.py caller updated.

Verified via uat/chartex_groundtruth_diff.py: histogram now matches the
PowerPoint-authored chart structurally (only delta = PowerPoint's optional
<cx:title>). Trinity: pytest 3887 passed 0 failed, ruff clean + fixed
point, behave 1109 scenarios 0 failed. Refs #14.

Author: MHoroszowski

src/pptx/oxml/chart/chartex.py

@@ -391,17 +391,21 @@ def add_box_whisker_series(
         return series
 
     def add_histogram_series(
-        self,
-        series_name: str,
-        series_name_ref: str = "Sheet1!$A$1",
-        data_id: int = 0,
-        bin_count: int | None = None,
-        bin_size: float | None = None,
+        self, series_name: str, series_name_ref: str = "Sheet1!$A$1", data_id: int = 0
     ):
         """Add a histogram series (`layoutId="clusteredColumn"` + `<cx:binning>`).
 
-        PowerPoint emits no `<cx:dataLabels>` for a histogram (ground-truth
-        diff, issue #14), so this series omits them.
+        Emits **automatic binning** — `<cx:binning intervalClosed="r"/>` with
+        no `binCount`/`binSize` child — which is exactly the structure
+        PowerPoint itself authors and accepts (verified via ground-truth diff,
+        issue #14). The `dml-chartex.xsd` models `binCount`/`binSize` as child
+        elements and our earlier emission was schema-valid, but PowerPoint's
+        reader rejects that form and shows a repair dialog. Until a
+        PowerPoint-authored sample with explicit bins exists to confirm the
+        accepted form, manual bin specification is intentionally not emitted —
+        PowerPoint computes sensible bins from the data automatically (its own
+        default behaviour). No `<cx:dataLabels>` either (PowerPoint emits none
+        for a histogram).
         """
         from lxml import etree
 
@@ -411,14 +415,6 @@ def add_histogram_series(
         layoutPr = etree.SubElement(series, qn("cx:layoutPr"))
         binning = etree.SubElement(layoutPr, qn("cx:binning"))
         binning.set("intervalClosed", "r")
-        # CT_Binning child is an xsd:choice of binSize | binCount (both optional →
-        # automatic binning, matching PowerPoint's default, when neither given).
-        if bin_size is not None:
-            bs = etree.SubElement(binning, qn("cx:binSize"))
-            bs.text = str(bin_size)
-        elif bin_count is not None:
-            bc = etree.SubElement(binning, qn("cx:binCount"))
-            bc.text = str(bin_count)
         return series
 
     def add_pareto_pair(

src/pptx/parts/slide.py

@@ -239,14 +239,9 @@ def add_chartex_part(self, chart_data: WaterfallChartData) -> str:
                 "val", chart_data.values_ref, chart_data.series_values, chart_data.number_format
             )
         elif cx_type == "histogram":
-            # numeric raw values, binned; no strDim, no dataLabels (PP ground truth)
-            plotAreaRegion.add_histogram_series(
-                name,
-                name_ref,
-                data_id=0,
-                bin_count=chart_data.bin_count,
-                bin_size=chart_data.bin_size,
-            )
+            # numeric raw values, auto-binned; no strDim, no dataLabels
+            # (PowerPoint ground truth, issue #14).
+            plotAreaRegion.add_histogram_series(name, name_ref, data_id=0)
             data_elem.add_numeric_dimension(
                 "val", chart_data.values_ref, chart_data.series_values, chart_data.number_format
             )