2D: Warn when axis added to multiple graphs

Adding the same axis to multiple graphs is not supported. It will result
to crash since graphsview destructor tries to set axis' associated graph
to nullptr.

Fixes: QTBUG-138822
Pick-to: 6.8 6.9 6.10
Change-Id: If306e415f87f2366ae0d82f6fd5c9a9a5e356f85
Reviewed-by: Kaj Grönholm <kaj.gronholm@qt.io>
Reviewed-by: Alexei Cazacov <alexei.cazacov@qt.io>

src/graphs2d/axis/qabstractaxis.cpp

@@ -625,11 +625,17 @@ QAbstractAxisPrivate::~QAbstractAxisPrivate()
         m_graph->removeAxis(q);
 }
 
+void QAbstractAxisPrivate::setGraph(QGraphsView *graph)
+{
+    if (m_graph && graph)
+        qCWarning(lcAxis2D, "%p axis already associated with %p", this, m_graph);
+
+    m_graph = graph;
+}
+
 void QAbstractAxisPrivate::handleRangeChanged(qreal min, qreal max)
 {
     setRange(min,max);
 }
 
 QT_END_NAMESPACE
-
-

src/graphs2d/axis/qabstractaxis_p.h

@@ -32,7 +32,7 @@ public:
     ~QAbstractAxisPrivate() override;
 
 public:
-    void setGraph(QGraphsView *graph) { m_graph = graph; }
+    void setGraph(QGraphsView *graph);
 
     //interface for manipulating range form base class
     virtual void setMin(const QVariant &min) = 0;

src/graphs2d/qgraphsview.cpp

@@ -1405,11 +1405,15 @@ void QGraphsView::updatePlotArea()
     \brief X-axis of this view.
 
     The x-axis used for the series inside this view.
+
+    \note Setting the same axis to multiple QGraphsViews is not supported.
 */
 /*!
     \qmlproperty AbstractAxis GraphsView::axisX
     The x-axis used for the series inside this view.
     \sa axisY
+
+    \note Setting the same axis to multiple GraphsViews is not supported.
 */
 
 QAbstractAxis *QGraphsView::axisX() const
@@ -1443,11 +1447,15 @@ void QGraphsView::setAxisX(QAbstractAxis *axis)
     \brief Y-axis of this view.
 
     The y-axis used for the series inside this view.
+
+    \note Setting the same axis to multiple QGraphsViews is not supported.
 */
 /*!
     \qmlproperty AbstractAxis GraphsView::axisY
     The y-axis used for the series inside this view.
     \sa axisX
+
+    \note Setting the same axis to multiple GraphsViews is not supported.
 */
 
 QAbstractAxis *QGraphsView::axisY() const

tests/auto/qml2dtest/axes/tst_valueaxis.qml

@@ -48,7 +48,21 @@ Item {
     }
 
     TestCase {
-        name: "ValueAxis Initial"
+        name: "ValueAxis with GraphsView"
+
+        GraphsView {id: graphsView1}
+        GraphsView {id: graphsView2}
+        ValueAxis {id: axis1}
+
+        function test_1_add_to_multiple() {
+            ignoreWarning(/.*axis already associated with.*/)
+            graphsView1.axisX = axis1
+            graphsView2.axisX = axis1
+        }
+    }
+
+    TestCase {
+        name: "valueaxis initial"
 
         Text { id: dummy }