[api] Set specific distance:elevation ratio for elevation profile canvas

python/PyQt6/gui/auto_generated/elevation/qgselevationprofilecanvas.sip.in

@@ -235,6 +235,35 @@ Sets whether the distance and elevation scales are locked to each other.
 .. seealso:: :py:func:`lockAxisScales`
 
 .. versionadded:: 3.32
+%End
+
+    double axisScaleRatio() const;
+%Docstring
+Returns the current ratio of horizontal (distance) to vertical
+(elevation) scale for the plot.
+
+.. seealso:: :py:func:`setAxisScaleRatio`
+
+.. versionadded:: 4.0
+%End
+
+    void setAxisScaleRatio( double scale );
+%Docstring
+Sets the ratio of horizontal (distance) to vertical (elevation) scale
+for the plot.
+
+E.g. a ``scale`` of 3 indicates a ratio of 3:1 for distance vs
+elevation, whereas a scale of 0.3333 indicates a ratio of 1:3 for
+distance vs elevation.
+
+This will immediately update the visible plot area to match the
+specified scale.
+
+.. seealso:: :py:func:`axisScaleRatio`
+
+.. seealso:: :py:func:`setLockAxisScales`
+
+.. versionadded:: 4.0
 %End
 
     Qgis::DistanceUnit distanceUnit() const;

python/gui/auto_generated/elevation/qgselevationprofilecanvas.sip.in

@@ -235,6 +235,35 @@ Sets whether the distance and elevation scales are locked to each other.
 .. seealso:: :py:func:`lockAxisScales`
 
 .. versionadded:: 3.32
+%End
+
+    double axisScaleRatio() const;
+%Docstring
+Returns the current ratio of horizontal (distance) to vertical
+(elevation) scale for the plot.
+
+.. seealso:: :py:func:`setAxisScaleRatio`
+
+.. versionadded:: 4.0
+%End
+
+    void setAxisScaleRatio( double scale );
+%Docstring
+Sets the ratio of horizontal (distance) to vertical (elevation) scale
+for the plot.
+
+E.g. a ``scale`` of 3 indicates a ratio of 3:1 for distance vs
+elevation, whereas a scale of 0.3333 indicates a ratio of 1:3 for
+distance vs elevation.
+
+This will immediately update the visible plot area to match the
+specified scale.
+
+.. seealso:: :py:func:`axisScaleRatio`
+
+.. seealso:: :py:func:`setLockAxisScales`
+
+.. versionadded:: 4.0
 %End
 
     Qgis::DistanceUnit distanceUnit() const;

src/gui/elevation/qgselevationprofilecanvas.cpp

@@ -605,16 +605,19 @@ void QgsElevationProfileCanvas::adjustRangeForAxisScaleLock( double &xMinimum, d
   // ensures that we always "zoom out" to match horizontal/vertical scales
   const double horizontalScale = ( xMaximum - xMinimum ) / mPlotItem->plotArea().width();
   const double verticalScale = ( yMaximum - yMinimum ) / mPlotItem->plotArea().height();
-  if ( horizontalScale > verticalScale )
+
+  const double currentRatio = horizontalScale / verticalScale;
+
+  if ( currentRatio <= mLockedAxisScale )
   {
-    const double height = horizontalScale * mPlotItem->plotArea().height();
+    const double height = horizontalScale * mPlotItem->plotArea().height() / mLockedAxisScale;
     const double deltaHeight = ( yMaximum - yMinimum ) - height;
     yMinimum += deltaHeight / 2;
     yMaximum -= deltaHeight / 2;
   }
   else
   {
-    const double width = verticalScale * mPlotItem->plotArea().width();
+    const double width = verticalScale * mPlotItem->plotArea().width() * mLockedAxisScale;
     const double deltaWidth = ( ( xMaximum - xMinimum ) - width );
     xMinimum += deltaWidth / 2;
     xMaximum -= deltaWidth / 2;
@@ -700,6 +703,32 @@ void QgsElevationProfileCanvas::setLockAxisScales( bool lock )
   }
 }
 
+double QgsElevationProfileCanvas::axisScaleRatio() const
+{
+  const double horizontalScale = ( mPlotItem->xMaximum() - mPlotItem->xMinimum() ) * mPlotItem->mXScaleFactor / mPlotItem->plotArea().width();
+  const double verticalScale = ( mPlotItem->yMaximum() - mPlotItem->yMinimum() ) / mPlotItem->plotArea().height();
+  return horizontalScale / verticalScale;
+}
+
+void QgsElevationProfileCanvas::setAxisScaleRatio( double scale )
+{
+  mLockedAxisScale = scale;
+
+  double xMinimum = mPlotItem->xMinimum() * mPlotItem->mXScaleFactor;
+  double xMaximum = mPlotItem->xMaximum() * mPlotItem->mXScaleFactor;
+  double yMinimum = mPlotItem->yMinimum();
+  double yMaximum = mPlotItem->yMaximum();
+  adjustRangeForAxisScaleLock( xMinimum, xMaximum, yMinimum, yMaximum );
+  mPlotItem->setXMinimum( xMinimum / mPlotItem->mXScaleFactor );
+  mPlotItem->setXMaximum( xMaximum / mPlotItem->mXScaleFactor );
+  mPlotItem->setYMinimum( yMinimum );
+  mPlotItem->setYMaximum( yMaximum );
+
+  refineResults();
+  mPlotItem->updatePlot();
+  emit plotAreaChanged();
+}
+
 QgsPointXY QgsElevationProfileCanvas::snapToPlot( QPoint point )
 {
   if ( !mCurrentJob || !mSnappingEnabled )

src/gui/elevation/qgselevationprofilecanvas.h

@@ -239,6 +239,29 @@ class GUI_EXPORT QgsElevationProfileCanvas : public QgsPlotCanvas
      */
     void setLockAxisScales( bool lock );
 
+    /**
+     * Returns the current ratio of horizontal (distance) to vertical (elevation) scale
+     * for the plot.
+     *
+     * \see setAxisScaleRatio()
+     * \since QGIS 4.0
+     */
+    double axisScaleRatio() const;
+
+    /**
+     * Sets the ratio of horizontal (distance) to vertical (elevation) scale for the plot.
+     *
+     * E.g. a \a scale of 3 indicates a ratio of 3:1 for distance vs elevation, whereas a scale
+     * of 0.3333 indicates a ratio of 1:3 for distance vs elevation.
+     *
+     * This will immediately update the visible plot area to match the specified scale.
+     *
+     * \see axisScaleRatio()
+     * \see setLockAxisScales()
+     * \since QGIS 4.0
+     */
+    void setAxisScaleRatio( double scale );
+
     /**
      * Returns the distance unit used by the canvas.
      *
@@ -340,6 +363,7 @@ class GUI_EXPORT QgsElevationProfileCanvas : public QgsPlotCanvas
     QgsScreenHelper *mScreenHelper = nullptr;
 
     bool mLockAxisScales = false;
+    double mLockedAxisScale = 1;
 
     QgsCoordinateReferenceSystem mCrs;
     QgsProject *mProject = nullptr;

tests/src/python/test_qgselevationprofilecanvas.py

@@ -225,6 +225,56 @@ class TestQgsElevationProfileCanvas(QgisTestCase):
         canvas.wheelEvent(wheel_event)
         self.assertEqual(tool.events[-1].type(), QEvent.Type.Wheel)
 
+    def test_ratio(self):
+        """
+        Test axis scale ratio logic
+        """
+        canvas = QgsElevationProfileCanvas()
+        canvas.setCrs(QgsCoordinateReferenceSystem("EPSG:4326"))
+        canvas.setFrameStyle(0)
+        # make a 2:1 canvas, to make the maths easier!
+        canvas.resize(800, 400)
+        canvas.setProject(QgsProject.instance())
+        canvas.show()
+        self.assertEqual(canvas.width(), 800)
+        self.assertEqual(canvas.height(), 400)
+
+        canvas.setVisiblePlotRange(100, 200, 50, 150)
+        # showing 100m distance, 100m elevation
+        # distance:elevation ratio is 1:2, as canvas is twice as wide as high
+        self.assertAlmostEqual(canvas.axisScaleRatio(), 0.5, 1)
+
+        canvas.setVisiblePlotRange(100, 300, 50, 150)
+        # showing 200m distance, 100m elevation
+        # distance:elevation ratio is 1:1, as canvas is twice as wide as high
+        self.assertAlmostEqual(canvas.axisScaleRatio(), 1.0, 1)
+
+        canvas.setVisiblePlotRange(100, 500, 50, 150)
+        # showing 400m distance, 100m elevation
+        # distance:elevation ratio is 2:1, as canvas is twice as wide as high
+        self.assertAlmostEqual(canvas.axisScaleRatio(), 2.0, delta=0.1)
+
+        canvas.setAxisScaleRatio(0.5)
+        self.assertAlmostEqual(canvas.axisScaleRatio(), 0.5, 1)
+        self.assertAlmostEqual(canvas.visibleDistanceRange().lower(), 248.024, 1)
+        self.assertAlmostEqual(canvas.visibleDistanceRange().upper(), 351.976, 1)
+        self.assertAlmostEqual(canvas.visibleElevationRange().lower(), 50.0, 1)
+        self.assertAlmostEqual(canvas.visibleElevationRange().upper(), 150.0, 1)
+
+        canvas.setAxisScaleRatio(1.0)
+        self.assertAlmostEqual(canvas.axisScaleRatio(), 1.0, 1)
+        self.assertAlmostEqual(canvas.visibleDistanceRange().lower(), 248.024, 1)
+        self.assertAlmostEqual(canvas.visibleDistanceRange().upper(), 351.976, 1)
+        self.assertAlmostEqual(canvas.visibleElevationRange().lower(), 75.0, 1)
+        self.assertAlmostEqual(canvas.visibleElevationRange().upper(), 125.0, 1)
+
+        canvas.setAxisScaleRatio(2.0)
+        self.assertAlmostEqual(canvas.axisScaleRatio(), 2.0, 1)
+        self.assertAlmostEqual(canvas.visibleDistanceRange().lower(), 248.024, 1)
+        self.assertAlmostEqual(canvas.visibleDistanceRange().upper(), 351.976, 1)
+        self.assertAlmostEqual(canvas.visibleElevationRange().lower(), 87.5, 1)
+        self.assertAlmostEqual(canvas.visibleElevationRange().upper(), 112.5, 1)
+
 
 if __name__ == "__main__":
     unittest.main()