Enbedding julia in C++ , how to call savefig function?

I am trying to use the julia C interface in orger to call Julia from within my C++ program.
I would like to save some plots but it does not seems to work.
Here is a snapshot of the code i used so far:

 // here used jl_main_module not like in the Julia's Doc !
	  jl_function_t *pl_sim  = jl_get_function(jl_main_module, "plot_model_sim");
	  jl_function_t *pl_data = jl_get_function(jl_main_module, "plot_model_data");
	  jl_module_t* PLOTS = (jl_module_t *)jl_eval_string("Plots");
	  jl_function_t *save = jl_get_function(PLOTS, "savefig");

	  std::cout << "\n julia:  found functions? "  << save << ":" << pl_sim  << ":" << pl_data << std::endl;
	  jl_value_t *a1    = jl_box_float64(params[0]);
	  jl_value_t *a2    = jl_box_float64(params[1]);
	  jl_value_t *a3    = jl_box_float64(params[2]);
	  jl_value_t *a4    = jl_box_float64(params[3]);
	  jl_value_t *a5    = jl_box_float64(params[4]);
	  jl_value_t *args[5]    = {a1 , a2, a3, a4, a5};
	  jl_value_t *plt_data     = jl_call(pl_data, args, 5);

	  jl_value_t *plt_sim      = jl_call(pl_sim, args, 5);

	  jl_value_t *title        = jl_eval_string(" \"output.png\" ");
          jl_value_t *out          = jl_call2(save, plt_sim, title);
	  std::cout << "\n julia:  outputs diagnostics "  << std::endl;

the call to savefig crashed with the output attached here.