pixel position of a point

[ferri_marllo gva es]

Hello!!
I have plot points in an X-Y plot using this code:


void grafica(GtkWidget *plot)
{
 GdkColor color;
 GtkPlotAxis *axis;
 int i,j,k,l;
 
 int np_x,np_y;
 np_x=0;
 np_y=0;
 
 
 t1a=g_array_new(FALSE,FALSE,sizeof(gdouble));
 t2a=g_array_new(FALSE,FALSE,sizeof(gdouble));
 v1a=g_array_new(FALSE,FALSE,sizeof(gdouble));
 v2a=g_array_new(FALSE,FALSE,sizeof(gdouble));
 

 
 matriz = &mat[0][0];   
 tiempo[FILAS] = mat[FILAS][0];
 var1[FILAS] = mat[FILAS][1];
 var2[FILAS] = mat[FILAS][2];
 
 
 fp=fopen(fichero_partida,"r");
 fp_sal=fopen("c:\\dev-cpp\\datos\\greca_matriz.txt","w");
 
 matriz=(double*)malloc(FILAS*COL*sizeof(double*));
 if(matriz==NULL)
 {
  g_print("No hay memoria");
 }
 
  
 for(i=0;i<FILAS;i++)
  { 
    for(j=0;j<COL;j++)
      {
        fscanf(fp,"%d",&mat[i][j]);
     
      }   
      
  }
 
 
   
  for(l=0;l<FILAS;l++)
  {
   tiempo[l]=mat[l][0];
   var1[l]=mat[l][1];
   var2[l]=mat[l][2];
  } 
  
  fprintf(fp_sal,"tiempo_1 %lf vari_2 %lf\n",tiempo[0],var1[1]);
  
    
  
   
  for(k=0;k<FILAS;k++)
  {
    
    if(tiempo[k]>=0.0 && var2[k]>=-50.0)
    {g_array_append_val(t2a,tiempo[k]);
      np_y=np_y+1;}
    
    if(var2[k]>=-50.0)
    {g_array_append_val(v2a,var2[k]);}
    if(tiempo[k]>=0.0 && var1[k]>=-50.0)
    {
     np_x=np_x+1;
         g_array_append_val(t1a,tiempo[k]);}
        if(var1[k]>=-50.0)
    {g_array_append_val(v1a,var1[k]);}
   }
   fprintf(fp_sal,"%d %d\n",np_x,np_y);
 
   
  
   
   t1=(gdouble*)t1a->data;
   t2=(gdouble*)t2a->data;
   v1=(gdouble*)v1a->data;  
   v2=(gdouble*)v2a->data;  
   
   g_array_free(t1a,FALSE);
   g_array_free(t2a,FALSE);
   g_array_free(v1a,FALSE);                                                     
        
   g_array_free(v2a,FALSE);   
         
 
    
    dataset[0] = GTK_PLOT_DATA(gtk_plot_data_new());
    gtk_plot_add_data(GTK_PLOT(plot), dataset[0]);
    gtk_widget_show(GTK_WIDGET(dataset[0]));
    
    
    gtk_plot_data_set_points(dataset[0], t1, v1,NULL,NULL,np_x);
    
    gdk_color_parse("red", &color);
    gdk_color_alloc(gdk_colormap_get_system(), &color); 

    gtk_plot_data_set_symbol(dataset[0],GTK_PLOT_SYMBOL_DIAMOND,
                          GTK_PLOT_SYMBOL_EMPTY,10, 2, &color, &color);
    gtk_plot_data_set_line_attributes(dataset[0],GTK_PLOT_LINE_SOLID,0, 0, 1,
&color);
                                
   

  
  fclose(fp);
  fclose(fp_sal);
     
}


but i'd like to know the pixel position of each plotted point in reference to
all the screen.
How can i Do it??

thanks a lot!