diff --git a/src/gui/control/graph.cpp b/src/gui/control/graph.cpp
index 56756e30..5542784a 100644
--- a/src/gui/control/graph.cpp
+++ b/src/gui/control/graph.cpp
@@ -85,9 +85,10 @@ GraphData::GraphData(const GraphDataPre& d)
 {
   // find groups on each axis
   size_t i = 0;
-  FOR_EACH(a, axes) {
+  for (auto const& a : axes) {
     map<String,UInt,SmartLess> counts; // note: default constructor for UInt() does initialize to 0
     FOR_EACH_CONST(e, d.elements) {
+      assert(e->values.size() == axes.size());
       counts[e->values[i]] += 1;
     }
     if (a->numeric) {
diff --git a/src/gui/control/graph.hpp b/src/gui/control/graph.hpp
index 6f69712b..adca76a5 100644
--- a/src/gui/control/graph.hpp
+++ b/src/gui/control/graph.hpp
@@ -28,7 +28,9 @@ DECLARE_LOCAL_EVENT_TYPE(EVENT_GRAPH_SELECT, <not used>)
 // ----------------------------------------------------------------------------- : Graph data
 
 /// A group in a table or graph
-/** A group is rendered as a single bar or pie slice */
+/** This corresponds to a specific value on an axis. For example "red" would be a GraphGroup for the "color" GraphAxis
+ *  A group is rendered as a single bar or pie slice.
+ */
 class GraphGroup : public IntrusivePtrBase<GraphGroup> {
 public:
   GraphGroup(const String& name, UInt size, const Color& color = *wxBLACK)
@@ -62,17 +64,17 @@ public:
     , order(order)
   {}
   
-  String             name;    ///< Name/label of this axis
-  AutoColor          auto_color;  ///< Automatically assign colors to the groups on this axis
-  vector<GraphGroup> groups;    ///< Groups along this axis
-  bool               numeric;    ///< Numeric axis?
-  double             bin_size;  ///< Group numeric values into bins of this size
-  UInt               max;      ///< Maximum size of the groups
-  UInt               total;    ///< Sum of the size of all groups
-  double             mean_value;    ///< Mean value, only for numeric axes
-  double             max_value;    ///< Maximal value, only for numeric axes
-  const map<String,Color>* colors;  ///< Colors for each choice (optional)
-  const vector<String>*    order;    ///< Order of the items (optional)
+  String                   name;         ///< Name/label of this axis
+  AutoColor                auto_color;   ///< Automatically assign colors to the groups on this axis
+  vector<GraphGroup>       groups;       ///< Groups along this axis
+  bool                     numeric;      ///< Numeric axis?
+  double                   bin_size;     ///< Group numeric values into bins of this size
+  UInt                     max;          ///< Maximum size of the groups
+  UInt                     total;        ///< Sum of the size of all groups
+  double                   mean_value;   ///< Mean value, only for numeric axes
+  double                   max_value;    ///< Maximal value, only for numeric axes
+  const map<String,Color>* colors;       ///< Colors for each choice (optional)
+  const vector<String>*    order;        ///< Order of the items (optional)
   
   /// Add a graph group
   void addGroup(const String& name, UInt size);
@@ -88,6 +90,8 @@ public:
 };
 
 /// Data to be displayed in a graph, not processed yet
+/** Requires: for (e : elements) e.values.size() == axes.size()
+ */
 class GraphDataPre {
 public:
   vector<GraphAxisP>    axes;
diff --git a/src/gui/set/stats_panel.cpp b/src/gui/set/stats_panel.cpp
index 99ec4df3..5978463b 100644
--- a/src/gui/set/stats_panel.cpp
+++ b/src/gui/set/stats_panel.cpp
@@ -477,7 +477,7 @@ void StatsPanel::showCategory(const GraphType* prefer_layout) {
   // find values for each card
   for (size_t i = 0 ; i < set->cards.size() ; ++i) {
     Context& ctx = set->getContext(set->cards[i]);
-    GraphElementP e(new GraphElement(i));
+    GraphElementP e = make_intrusive<GraphElement>(i);
     bool show = true;
     FOR_EACH(dim, dims) {
       try {
@@ -490,9 +490,12 @@ void StatsPanel::showCategory(const GraphType* prefer_layout) {
         }
       } catch (ScriptError const& e) {
         handle_error(ScriptError(e.what() + _("\n  in script for statistics dimension '") + dim->name + _("'")));
+        show = false;
+        break;
       }
     }
     if (show) {
+      assert(e->values.size() == dims.size());
       d.elements.push_back(e);
     }
   }