oox: a scatter's numeric X values export as numbers

A scatter chart whose X values come from its categories wrote them as
a string reference (strRef) to PPTX. PowerPoint and LibreOffice read
string X values as point indices, so X became 1, 2, 3 after a round
trip. When every category is a number, write a numRef instead.

diff --git a/oox/source/export/chartexport.cxx b/oox/source/export/chartexport.cxx
index 3947280..03f3d78 100644
--- a/oox/source/export/chartexport.cxx
+++ b/oox/source/export/chartexport.cxx
@@ -4066,6 +4066,16 @@ void ChartExport::exportSeriesCategory( const Reference< chart2::data::XDataSequ
             if (aNumberFormatString.isEmpty())
                 bWriteDateCategories = false;
         }
+        // Numeric X values stay numbers.
+        if (nValueType == XML_xVal)
+        {
+            const std::vector<double> aX = lcl_getAllValuesFromSequence(xValueSeq);
+            if (!aX.empty() && std::none_of(aX.begin(), aX.end(), [](double f) { return std::isnan(f); }))
+            {
+                bWriteDateCategories = true;
+                aNumberFormatString = u"General"_ustr;
+            }
+        }
 
         pFS->startElement(FSNS(XML_c, bWriteDateCategories ? XML_numRef : XML_strRef));