Remove 'native data' and 'native units' (#331)

Resolves #331 (closed)

Somehow resolves #850 (closed)

GUI/Model/Job/JobItem.cpp

@@ -301,11 +301,12 @@ DataItem* JobItem::createDiffDataItem()
     ASSERT(!diffDataItem());
     m_diffDataItem.reset(createNewDataItem());
 
-    // use the same axes as real item
     ASSERT(m_realItem);
-    GUI::Util::copyContents(m_realItem->dataItem()->axItemX(), m_diffDataItem->axItemX());
+    // use the same axes as simulated item (same as external data item if loading pre-22.0 project)
+    const DataItem* source = m_simulatedDataItem ? simulatedDataItem() : m_realItem->dataItem();
+    GUI::Util::copyContents(source->axItemX(), m_diffDataItem->axItemX());
     if (rank() == 2)
-        GUI::Util::copyContents(m_realItem->dataItem()->axItemY(), m_diffDataItem->axItemY());
+        GUI::Util::copyContents(source->axItemY(), m_diffDataItem->axItemY());
 
     if (auto* spec_diff = dynamic_cast<Data1DItem*>(diffDataItem()))
         spec_diff->setDiffPlotStyle();