... I found ... a newer version of the Cartesian library here: https://sourceforge.net/p/rfltk/code/HEAD/tree/cartesian/

Meanwhile I also found out that this library adds more than one FLTK widget to its parent group (or window) - which may cause the double free under certain circumstances - and I could indeed make the provided example program crash with double free if I only delete the main window after `Fl::run()`.

Source code: https://sourceforge.net/p/rfltk/code/HEAD/tree/cartesian/test/example.cpp

Modified like this (see comment):
```

Fl_Double_Window *w = new Fl_Double_Window(580, 380, "Cartesian graphics example");

// ... more code here ...

w->end();

w->show(argc, argv);

Fl::add_timeout(0, next_freq);

Fl::run();

delete w; // ADDED only this statement

return 0;

```

This is an example I can work with, and I'll investigate this issue. ...

I'll let you know when I have more info.

Cartesian.cpp-728-Ca_X_Axis::~Ca_X_Axis(){
Cartesian.cpp-729-  if(canvas_){
Cartesian.cpp-730-    Ca_ObjectChain *ochain=canvas_->first_object_;
Cartesian.cpp-731-    Ca_ObjectChain *next;
Cartesian.cpp-732-    Ca_ObjectChain *previous=0;
Cartesian.cpp-733-    while (ochain){
Cartesian.cpp-734-      next=ochain->next;
Cartesian.cpp-735-      if(ochain->object->x_axis_==this){
Cartesian.cpp-736-        delete ochain->object;
Cartesian.cpp-737-        if(previous)
Cartesian.cpp-738-          previous->next=next;
Cartesian.cpp-739-        else
Cartesian.cpp-740-          canvas_->first_object_=next;
Cartesian.cpp:741:        // delete ochain; // would cause double free        <<<<<<<
Cartesian.cpp-742-      }
Cartesian.cpp-743-      ochain=next;
Cartesian.cpp-744-    }
Cartesian.cpp-745-  }
Cartesian.cpp-746-}
--
Cartesian.cpp-1051-Ca_Y_Axis::~Ca_Y_Axis(){
Cartesian.cpp-1052-  if(canvas_){
Cartesian.cpp-1053-    Ca_ObjectChain *ochain=canvas_->first_object_;
Cartesian.cpp-1054-    Ca_ObjectChain *next;
Cartesian.cpp-1055-    Ca_ObjectChain *previous=0;
Cartesian.cpp-1056-    while (ochain){
Cartesian.cpp-1057-      next=ochain->next;
Cartesian.cpp-1058-      if(ochain->object->y_axis_==this){
Cartesian.cpp-1059-        delete ochain->object;
Cartesian.cpp-1060-        if(previous)
Cartesian.cpp-1061-          previous->next=next;
Cartesian.cpp-1062-        else
Cartesian.cpp-1063-          canvas_->first_object_=next;
Cartesian.cpp:1064:        // delete ochain; // would cause double free        <<<<<<<
Cartesian.cpp-1065-      }
Cartesian.cpp-1066-      ochain=next;
Cartesian.cpp-1067-    }
Cartesian.cpp-1068-  }
Cartesian.cpp-1069-}