function phaseplane(command_str)
% PHASED
%

if nargin == 0
  command_str = 'initialize';
end

global h_box h_grid h_xdata h_yprimedata h_status h_n h_clear

if strcmp(command_str,'initialize')

    % Make sure that the GUI has not been initialized
    % in another existing figure.
    h_figs = get(0,'children');
    fig_exists = 0;
    for fig = h_figs'
      fig_exists = strcmp(get(fig,'name'),'Phase Plane Trajectories');
      if fig_exists
         figure(fig);  % Bring figure to front of screen
         return;       % No need to reinitialize, exit function
      end
    end    
   
    
    h_fig = figure('name','Phase Plane Trajectories');

        axes('position',[.2 .3 .6 .6],'box','on')

        % Create the two frames.
        uicontrol(h_fig,... 
                'Position',[ 0 0 1 0.25 ],... 
                'Style','frame',... 
                'BackgroundColor', [.702, .702, .702],...
                'Units','normalized'); 

       

             % Create the Edit boxes for the x data
             % and data box for number of trajectories plotted
        h_xdata = uicontrol(h_fig,... 
                'CallBack','phaseplane(''Plot Function'');',... 
                'Position',[ 0.25 0.15 0.15 0.06 ],... 
                'String','-5:1:5',... 
                'Style','edit',... 
                'Units','normalized'); 
        h_n = uicontrol(h_fig,... 
                'CallBack','phaseplane(''Plot Trajectory'');',... 
                'Position',[ 0.7 0.15 0.25 0.06 ],... 
                'String','0|5|10|15|20', ... 
                'Style','listbox',... 
                'Units','normalized',...
                'Value',[1]); 
		h_clear = uicontrol(h_fig,... 
                'CallBack','phaseplane(''Clear Axis'');',... 
                'Position',[ 0.7 0.08 0.25 0.06 ],... 
                'String','Clear', ... 
                'Style','pushbutton',... 
                'Units','normalized');

   

        % Create a static text object that will be used
        % to display messages to the user.
        %h_status = uicontrol(h_fig,... 
         %       'Position',[ 0.1 0.06 0.8 0.05 ],... 
          %      'String','Status Window',... 
           %     'Style','text',... 
            %    'Units','normalized'); 
        % Create the "x = " and "y(x)=" static text objects
        uicontrol(h_fig,... 
                'Position',[ 0.1 0.15 0.12 0.06 ],... 
                'String','x =',... 
                'Style','text',... 
                'Units','normalized'); 
        uicontrol(h_fig,... 
                'Position',[ 0.5 0.15 0.12 0.06 ],... 
                'String','number of trajectories = ',... 
                'Style','text',... 
                'Units','normalized'); 


        % Initialize the plot with the initial x and y data
       phaseplane('Plot Function');


        
% Callback for x,y,yprime edit boxes
            
elseif strcmp(command_str,'Plot Function')
   
   		err_ind = 0;
        eval(['x = ' get(h_xdata,'string') ';'],'err_ind=1;');
     
     
     y=x;
     
 

        if err_ind == 0
				dx=x(2)-x(1);
				dy=dx;
				nx=length(x);
				ny=length(y);
				l=.5*dx;
				for i=1:nx
  					for j=1:ny
  							v=[x(i);y(j)];
  							m=g(0,v);
  							m1=m(1);
  							m2=m(2);
						%  len=1/sqrt(m1^2+m2^2);
  					if (m1~=0) 
    						 s=m2/m1;
  					else
     						s=10^5;
 					 end
  
  					xstar=x(i)-l/sqrt(1+s^2);
  					ystar=y(j)+s*(xstar-x(i));
  
 					 u=[x(i), xstar];
  					v=[y(j),ystar];
  
  					plot(u,v)
  					hold on
  
  					end
					end
			axis([x(1), x(end), y(1), y(end)]);
			hold off;
      end

          
                    
         
 % Callback for trajectory plot
 %
elseif strcmp(command_str,'Plot Trajectory')
        err_ind = 0;
        no_traj = get(h_n,'value');
%        eval(['n = ' get(h_n,'string') ';'],'err_ind=1;');
        hold on
        for i=1:5*(no_traj-1)    
                       
           [y10,y20]=ginput(1);
           [T,Y]=ode45('g',[0,10],[y10,y20]);
           plot(Y(:,1),Y(:,2),'r')
           hold on
           [T,Y]=ode45('g',[0, -10],[y10,y20]);
           plot(Y(:,1),Y(:,2),'m')

           i=i+1;
        end
        hold off

 elseif strcmp(command_str,'Clear Axis')
      cla;
      phaseplane('Plot Function');
end
   

        
%end % END command_str checks