GUI Plot using Timer Object and For Loops

I am trying to plot an equation in a GUI using timer object as as the variable for the x-axis. When I plot WITHOUT the for-loop, the GUI will plot a nice line while running continuously (to an infinite time or a large time 't') and you can see the red circle on the right edge of the plot. However I need to use the for-loop to correctly simulate the equation under given if/else conditions, otherwise it will just neglect the 'else' statement. The problem is that although the GUI will plot with the for-loop correctly, it seems to overwrite the existing data. Thus I won't see a curve profile of the equation but instead just a red circle on the right edge showing where the plot should be as time increases. How do I plot a GUI using timer objects with a for-loop and have it plot a nice curve without overwriting existing data to plot a nice single curve? (Perhaps using something similar to the "hold on" command?)

Below is the code for my timer callback functions that does all the calculations and plotting. I attached the GUI .fig and .m files for convenience

    % code
function user_timercallback(obj,event,hObject)
    handles=guidata(hObject);
    handles.t=handles.t+0.01;

    % Initial Conditions
    Pin = 1;                    % Input Pressure (Pa) - Controlled by User
    P1 = 0;                     % Initial Pressure 1 (Pa)
    r0 = 0.0254;                % Initial chamber radius (m)
    dP1_shutoff = 1.2;          % Shutoff Pressure (Pa)
    Rst = 1;                    % Flow Resistance
    V1 = (4/3)*pi*r0^3;         % Initial Volume (m^3)

    % Ideal Gas Law Variables
    dn = 0.0000001;             % moles/sec
    R = 8.3145;                 % J/(mol K)
    T = 293.15;                 % K

    % Calculation of Pressure 1 with constant volume
    for t=0:0.01:handles.t
        if -dP1_shutoff < Pin - P1
            P1 = (dn.*t.*R*T)./V1
        else
            Vdot = 0;
        end
%     % Calculation of Pressure 1 with varying volume
%     for t=0:0.01:handles.t;
%         if -dP1_shutoff < Pin - P1
%             Vdot = (Pin - P1)./R;
%         else
%             Vdot = 0;
%         end
%         
%     V1 = V1 + Vdot*t;                           % Updated volume of the chamber
%     r = ((3./(4.*pi)).*V1).^(1./3);             % Updated radius of the chamber
%     P1 = 2*[1./(r0.^2.*r)].*[1-(r0./r).^6]      % Updated Pressure 1

    x = t;
    y = P1;

    set(handles.axes_myax,'xlim',[0 handles.t]); 
    set(handles.signal,'xdata',t,'ydata',y);
    set(handles.dot,'xdata',t(end),'ydata',y(end));
    guidata(hObject,handles);
    end

Answer 1

[Geoff Hayes]

Gary - note how you update the line object for the signal as

 set(handles.signal,'xdata',t,'ydata',y);

where t and y are initialized as scalars. So the existing XData and YData is replaced with the new value and all previous history is lost. You would need to do something like

 xData = [get(handles.signal,'XData') x];
 yData = [get(handles.signal,'YData') y];

 set(handles.axes_myax,'xlim',[0 handles.t]);    
 set(handles.signal,'xdata',xData,'ydata',yData);

to ensure that the history is preserved.

I did notice a potential problem. There is a for loop in the timer callback. Is this intentional because it seems to pollute the x data as it is constantly reset to zero (due to the loop) and so we have "waves" in the data.

[Gary Chen]

Ok I will try that. As for the for loop in the timer callback, I am using the for loop so that the GUI will solve the equation correctly with the given if/else conditions. My intention was to have the user be able to control the P1 parameter using a slider while the GUI is running (This i completed successfully just recently). The "waves" created when the GUI runs is due to the constant P1 that I set for testing purposes. Other than that it does cause a problem when the GUI runs over a long duration as the GUI starts to starts to slow down.

[Gary Chen]

Ok after running the program, I see what you mean by the plot resetting to 0. Is there a way for the plot to not reset? Like some sort of "hold on" command for a GUI?

[Geoff Hayes]

Your timer is firing every 0.01 seconds. What do you want to happen every time this timer fires? Do you want to start over or from where you last left off? Does the 0.01 correspond to the step size in your for loop as both are identical?

[Gary Chen]

I want it to plot right where I left off. And yes 0.01 should be my be my step size.

[Geoff Hayes]

Then you don't need the for loop. Since you last ended at handles.t, then use handles.t + 0.01 for the current timer callback.

[Gary Chen]

I actually found the problem. I had the "end" command at the wrong place. it should be right after the if/else commands and not at the very end of the function