don't create a scan just to obtain n

GUI/Model/Device/GrazingScanItem.cpp

@@ -148,6 +148,11 @@ double GrazingScanItem::scaleFactor() const
     return Units::deg;
 }
 
+int GrazingScanItem::nBins() const
+{
+    return alphaAxisItem()->binCount();
+}
+
 BasicAxisItem* GrazingScanItem::alphaAxisItem() const
 {
     return m_currentAxisIsUniformAxis ? m_uniformAlphaAxis.get() : m_pointwiseAlphaAxis.get();

GUI/Model/Device/GrazingScanItem.h

@@ -36,6 +36,8 @@ public:
 
     double scaleFactor() const override;
 
+    int nBins() const;
+
     //! The currently selected axis
     BasicAxisItem* alphaAxisItem() const;
 

GUI/Model/Device/InstrumentItems.cpp

@@ -575,7 +575,7 @@ OffspecInstrumentItem::OffspecInstrumentItem()
 
 std::vector<int> OffspecInstrumentItem::shape() const
 {
-    return {(int)createScan()->nScan(), detectorItem()->ySize()};
+    return {scanItem()->grazingScanItem()->nBins(), detectorItem()->ySize()};
 }
 
 void OffspecInstrumentItem::updateToRealData(const RealItem* dataItem)