Also fix pretty-breaks when single class

src/core/classification/qgsclassificationprettybreaks.cpp

@@ -45,6 +45,13 @@ QList<double> QgsClassificationPrettyBreaks::calculateBreaks( double &minimum, d
   if ( symmetricModeEnabled() )
     makeBreaksSymmetric( breaks, symmetryPoint(), symmetryAstride() );
 
+  // Special case for single class
+  if ( minimum == maximum && breaks.isEmpty() )
+  {
+    // 1 is totally arbitrary but we need something
+    breaks << maximum + 1.0;
+  }
+
   return breaks;
 }
 

tests/src/python/test_qgsgraduatedsymbolrenderer.py

@@ -26,6 +26,8 @@ from qgis.core import (
     QgsRendererRangeLabelFormat,
     QgsVectorLayer,
     QgsFillSymbol,
+    QgsClassificationFixedInterval,
+    QgsClassificationPrettyBreaks,
 )
 import unittest
 from qgis.testing import start_app, QgisTestCase
@@ -799,6 +801,35 @@ class TestQgsGraduatedSymbolRenderer(QgisTestCase):
             "Equal interval classification not correct",
         )
 
+    def testPrettyBreakSingleClass(self):
+        """Test single class with pretty break, issue GH #63277"""
+
+        # Create a renderer
+        renderer = QgsGraduatedSymbolRenderer()
+        renderer.setClassificationMethod(QgsClassificationPrettyBreaks())
+        symbol = createMarkerSymbol()
+        renderer.setSourceSymbol(symbol.clone())
+
+        # Test retrieving data values from a layer
+        ml = createMemoryLayer((0, 0))
+
+        renderer.setClassAttribute("value")
+        # Equal interval calculations
+        renderer.updateClasses(ml, 5)
+        self.assertEqual(
+            dumpRangeBreaks(renderer.ranges()),
+            "(0.0000-1.0000,)",
+            "Equal interval classification not correct",
+        )
+
+        ml = createMemoryLayer((1, 1))
+        renderer.updateClasses(ml, 5)
+        self.assertEqual(
+            dumpRangeBreaks(renderer.ranges()),
+            "(1.0000-2.0000,)",
+            "Equal interval classification not correct",
+        )
+
 
 if __name__ == "__main__":
     unittest.main()