diff --git a/python/core/diagram/qgsdiagram.sip b/python/core/diagram/qgsdiagram.sip
index 58695eeacb0..0df562c1190 100644
--- a/python/core/diagram/qgsdiagram.sip
+++ b/python/core/diagram/qgsdiagram.sip
@@ -10,7 +10,15 @@ class QgsDiagram
     virtual QgsDiagram* clone() const = 0;
 
     void clearCache();
-    QgsExpression* getExpression( const QString& expression, const QgsFields* fields );
+    QgsExpression* getExpression( const QString& expression, const QgsFields* fields ) /Deprecated/;
+
+    /** Returns a prepared expression for the specified context.
+     * @param expression expression string
+     * @param context expression context
+     * @note added in QGIS 2.12
+     */
+    QgsExpression* getExpression( const QString& expression, const QgsExpressionContext& context );
+
     /** @deprecated `void renderDiagram( const QgsFeature& feature, QgsRenderContext& c, const QgsDiagramSettings& s, const QPointF& position )` should be used instead */
     virtual void renderDiagram( const QgsAttributes& att, QgsRenderContext& c, const QgsDiagramSettings& s, const QPointF& position ) /Deprecated/;
     /** Draws the diagram at the given position (in pixel coordinates)*/
diff --git a/src/core/diagram/qgsdiagram.cpp b/src/core/diagram/qgsdiagram.cpp
index 574cf4d79d6..332d2fcabfe 100644
--- a/src/core/diagram/qgsdiagram.cpp
+++ b/src/core/diagram/qgsdiagram.cpp
@@ -46,6 +46,7 @@ void QgsDiagram::clearCache()
 
 QgsExpression* QgsDiagram::getExpression( const QString& expression, const QgsFields* fields )
 {
+  Q_NOWARN_DEPRECATED_PUSH
   if ( !mExpressions.contains( expression ) )
   {
     QgsExpression* expr = new QgsExpression( expression );
@@ -53,6 +54,18 @@ QgsExpression* QgsDiagram::getExpression( const QString& expression, const QgsFi
     mExpressions[expression] = expr;
   }
   return mExpressions[expression];
+  Q_NOWARN_DEPRECATED_POP
+}
+
+QgsExpression *QgsDiagram::getExpression( const QString &expression, const QgsExpressionContext &context )
+{
+  if ( !mExpressions.contains( expression ) )
+  {
+    QgsExpression* expr = new QgsExpression( expression );
+    expr->prepare( &context );
+    mExpressions[expression] = expr;
+  }
+  return mExpressions[expression];
 }
 
 void QgsDiagram::setPenWidth( QPen& pen, const QgsDiagramSettings& s, const QgsRenderContext& c )
diff --git a/src/core/diagram/qgsdiagram.h b/src/core/diagram/qgsdiagram.h
index c61c4917b1f..f77bfa125d8 100644
--- a/src/core/diagram/qgsdiagram.h
+++ b/src/core/diagram/qgsdiagram.h
@@ -16,6 +16,7 @@
 #define QGSDIAGRAM_H
 
 #include "qgsfeature.h"
+#include "qgsexpressioncontext.h"
 #include <QPen>
 #include <QBrush>
 
@@ -40,7 +41,16 @@ class CORE_EXPORT QgsDiagram
     virtual QgsDiagram* clone() const = 0;
 
     void clearCache();
-    QgsExpression* getExpression( const QString& expression, const QgsFields* fields );
+
+    Q_DECL_DEPRECATED QgsExpression* getExpression( const QString& expression, const QgsFields* fields );
+
+    /** Returns a prepared expression for the specified context.
+     * @param expression expression string
+     * @param context expression context
+     * @note added in QGIS 2.12
+     */
+    QgsExpression* getExpression( const QString& expression, const QgsExpressionContext& context );
+
     /** @deprecated `void renderDiagram( const QgsFeature& feature, QgsRenderContext& c, const QgsDiagramSettings& s, const QPointF& position )` should be used instead */
     virtual Q_DECL_DEPRECATED void renderDiagram( const QgsAttributes& att, QgsRenderContext& c, const QgsDiagramSettings& s, const QPointF& position );
     /** Draws the diagram at the given position (in pixel coordinates)*/
diff --git a/src/core/diagram/qgshistogramdiagram.cpp b/src/core/diagram/qgshistogramdiagram.cpp
index 022017d4ae9..d2650be31cc 100644
--- a/src/core/diagram/qgshistogramdiagram.cpp
+++ b/src/core/diagram/qgshistogramdiagram.cpp
@@ -37,7 +37,6 @@ QgsDiagram* QgsHistogramDiagram::clone() const
 
 QSizeF QgsHistogramDiagram::diagramSize( const QgsFeature& feature, const QgsRenderContext& c, const QgsDiagramSettings& s, const QgsDiagramInterpolationSettings& is )
 {
-  Q_UNUSED( c );
   QSizeF size;
   if ( feature.attributes().count() == 0 )
   {
@@ -49,10 +48,15 @@ QSizeF QgsHistogramDiagram::diagramSize( const QgsFeature& feature, const QgsRen
 
   double maxValue = 0;
 
+  QgsExpressionContext expressionContext = c.expressionContext();
+  expressionContext.setFeature( feature );
+  if ( feature.fields() )
+    expressionContext.setFields( *feature.fields() );
+
   foreach ( QString cat, s.categoryAttributes )
   {
-    QgsExpression* expression = getExpression( cat, feature.fields() );
-    maxValue = qMax( expression->evaluate( feature ).toDouble(), maxValue );
+    QgsExpression* expression = getExpression( cat, expressionContext );
+    maxValue = qMax( expression->evaluate( &expressionContext ).toDouble(), maxValue );
   }
 
   // Scale, if extension is smaller than the specified minimum
@@ -126,10 +130,15 @@ void QgsHistogramDiagram::renderDiagram( const QgsFeature& feature, QgsRenderCon
   QList<double> values;
   double maxValue = 0;
 
+  QgsExpressionContext expressionContext = c.expressionContext();
+  expressionContext.setFeature( feature );
+  if ( feature.fields() )
+    expressionContext.setFields( *feature.fields() );
+
   foreach ( QString cat, s.categoryAttributes )
   {
-    QgsExpression* expression = getExpression( cat, feature.fields() );
-    double currentVal = expression->evaluate( feature ).toDouble();
+    QgsExpression* expression = getExpression( cat, expressionContext );
+    double currentVal = expression->evaluate( &expressionContext ).toDouble();
     values.push_back( currentVal );
     maxValue = qMax( currentVal, maxValue );
   }
diff --git a/src/core/diagram/qgspiediagram.cpp b/src/core/diagram/qgspiediagram.cpp
index 56f44392376..ae6cfdf9e1b 100644
--- a/src/core/diagram/qgspiediagram.cpp
+++ b/src/core/diagram/qgspiediagram.cpp
@@ -42,8 +42,13 @@ QSizeF QgsPieDiagram::diagramSize( const QgsFeature& feature, const QgsRenderCon
   QVariant attrVal;
   if ( is.classificationAttributeIsExpression )
   {
-    QgsExpression* expression = getExpression( is.classificationAttributeExpression, feature.fields() );
-    attrVal = expression->evaluate( feature );
+    QgsExpressionContext expressionContext = c.expressionContext();
+    if ( feature.fields() )
+      expressionContext.setFields( *feature.fields() );
+    expressionContext.setFeature( feature );
+
+    QgsExpression* expression = getExpression( is.classificationAttributeExpression, expressionContext );
+    attrVal = expression->evaluate( &expressionContext );
   }
   else
   {
@@ -121,11 +126,16 @@ void QgsPieDiagram::renderDiagram( const QgsFeature& feature, QgsRenderContext&
   double valSum = 0;
   int valCount = 0;
 
+  QgsExpressionContext expressionContext = c.expressionContext();
+  expressionContext.setFeature( feature );
+  if ( feature.fields() )
+    expressionContext.setFields( *feature.fields() );
+
   QList<QString>::const_iterator catIt = s.categoryAttributes.constBegin();
   for ( ; catIt != s.categoryAttributes.constEnd(); ++catIt )
   {
-    QgsExpression* expression = getExpression( *catIt, feature.fields() );
-    currentVal = expression->evaluate( feature ).toDouble();
+    QgsExpression* expression = getExpression( *catIt, expressionContext );
+    currentVal = expression->evaluate( &expressionContext ).toDouble();
     values.push_back( currentVal );
     valSum += currentVal;
     if ( currentVal ) valCount++;
diff --git a/src/core/qgsvectorlayerrenderer.cpp b/src/core/qgsvectorlayerrenderer.cpp
index 9e0c0d49eb1..2d5f49e80eb 100644
--- a/src/core/qgsvectorlayerrenderer.cpp
+++ b/src/core/qgsvectorlayerrenderer.cpp
@@ -520,7 +520,7 @@ void QgsVectorLayerRenderer::prepareDiagrams( QgsVectorLayer* layer, QStringList
   QList<QString>::const_iterator attIt = att.constBegin();
   for ( ; attIt != att.constEnd(); ++attIt )
   {
-    QgsExpression* expression = diagRenderer->diagram()->getExpression( *attIt, &mFields );
+    QgsExpression* expression = diagRenderer->diagram()->getExpression( *attIt, mContext.expressionContext() );
     QStringList columns = expression->referencedColumns();
     QStringList::const_iterator columnsIterator = columns.constBegin();
     for ( ; columnsIterator != columns.constEnd(); ++columnsIterator )
@@ -535,7 +535,7 @@ void QgsVectorLayerRenderer::prepareDiagrams( QgsVectorLayer* layer, QStringList
   {
     if ( linearlyInterpolatedDiagramRenderer->classificationAttributeIsExpression() )
     {
-      QgsExpression* expression = diagRenderer->diagram()->getExpression( linearlyInterpolatedDiagramRenderer->classificationAttributeExpression(), &mFields );
+      QgsExpression* expression = diagRenderer->diagram()->getExpression( linearlyInterpolatedDiagramRenderer->classificationAttributeExpression(), mContext.expressionContext() );
       QStringList columns = expression->referencedColumns();
       QStringList::const_iterator columnsIterator = columns.constBegin();
       for ( ; columnsIterator != columns.constEnd(); ++columnsIterator )