addpoints error "Value must be a handle"

Aim: Continuously read & plot data acquired from Arduino Uno + pressure transducer setup using "animatedline". After the experiment is done, hit "STOP" button on GUI, and gather data collected using "getpoints" to then be processed as necessary.

Problem: When I run the code, I am able to plot live signal from my transducer using "animatedline" (illustrated below).

The code that enables this is the following:

figure 
an = animatedline('Marker','.','Color','r','LineWidth',1.5);
ax = gca; box on
ax.XGrid = 'on'; ax.YGrid = 'on'; 
ax.YLim = [0 32]; % psi

ButtonHandle = uicontrol('Style', 'PushButton', ...
                         'String', 'STOP', ...
                         'BackgroundColor','r',...
                         'Callback', 'exit_loop');

startTime = datetime('now');
eject = 0;
while t < 10
    voltage = readVoltage(a,'A0');
    maxP = 30; minP = 0; % Dictated by transducer
    pOffset = 7.44; % Observed voltage correction
    pressure = voltage*((maxP-minP)/(5-1))-pOffset; % Convert signal to pressure

    t = datetime('now') - startTime;  
    addpoints(an,datenum(t),pressure) % adds points to plot
    ax.XLim = datenum([t-seconds(15) t]); % updates the x-axis
    datetick('x','keeplimits') % change x-axis to time label
    
    drawnow limitrate
end

The code for the "STOP" button in the bottom left corner is below. I have this saved as a separate script, but in the same folder:

function exit_loop(hObject, eventdata, handles)
    
selection = questdlg('Sure you want to exit?',...
                          'Close Request Function',...
                          'Yes','No','Yes');
     switch selection
        case 'Yes'
           delete(gcf)
           
        case 'No'
           return 
           
     end

Once I hit "Yes," however, I then get this error:

Error using matlab.graphics.animation.AnimatedLine/addpoints
Value must be a handle.

Error in pRecord_v0 (line 117)
    addpoints(an,datenum(t),pressure) % adds
    points to plot
    

Same error when I try to store the data from the experiment using the following:

[timeLog,pLog] = getpoints(an);

Any help you can offer is much appreciated!

Answer 1

[Steven Lord]

Guard your addpoints call with a check to make sure the animatedline handle still isvalid.

L = animatedline;
axis([0 360 -1 1]);
for d = 0:360
    if isvalid(L)
        addpoints(L, d, sind(d));
    else
        break % stop trying to add points
    end
    if d == 180
        cla % get rid of the animatedline
    end
    drawnow expose
end
disp(d) % 181

[Luke G.]

This was a great suggestion. Thank you! I don't understand how it fixed the problem though - could you please explain?

Also, while it eliminated the error after collection. I still got the same error when I tried to post-process the data. So, I just copy-and-pasted that snipped of code into the while loop--allowing me to log data within the while loop. See the below code, and thanks again!

figure 
an = animatedline('Marker','.','Color','r','LineWidth',1.5);
ax = gca; box on
ax.XGrid = 'on'; ax.YGrid = 'on'; 
ax.YLim = [0 32]; % psi

if pOffset == 0
    checkStr = 'Your pressure offset = 0.\n Are you sure you want to continue?';
    warnMe = questdlg();
end

ButtonHandle = uicontrol('Style', 'PushButton', ...
                         'String', 'STOP', ...
                         'BackgroundColor','r',...
                         'Callback', 'exit_loop');

startTime = datetime('now');
eject = 0;
while t < 10
    voltage = readVoltage(a,'A0');
    maxP = 30; minP = 0; % Dictated by transducer
    pOffset = 7.44; % Observed voltage correction
    pressure = voltage*((maxP-minP)/(5-1))-pOffset; % Convert signal to pressure

    t = datetime('now') - startTime;
    
    if isvalid(an)
        addpoints(an,datenum(t),pressure) % adds points to plot
        [timeLog,pLog] = getpoints(an); % logging the data
    else
        break
    end
    
    ax.XLim = datenum([t-seconds(15) t]); % updates the x-axis
    datetick('x','keeplimits') % change x-axis to time label
    drawnow limitrate % refreshes window
end


%% POST-EXPERIMENT PLOT 
timeSecs = (timeLog-timeLog(1))*24*3600;

smoothedPressure = smooth(pLog,25);
pPlotMax = smoothedPressure + 0.075; % Dwyer 626 has full scale accuracy of 0.25%
pPlotMin = smoothedPressure - 0.075; 

figure
hold on
plot(timeSecs,pLog,'.k',...
     timeSecs,smoothedPressure,'r',...
     timeSecs,pPlotMax,'r--',...
     timeSecs,pPlotMin,'r--')
legend('Raw','Smoothed');
xlabel('Elapsed time (sec)')
ylabel('Pressure (psi)')
hold off; box on

[Steven Lord]

Let's say you're writing down numbers in a notebook once an hour. Then between writing two numbers, someone throws your notebook into a paper shredder. When you have to record the next number you write the number on the table, look around for your notebook, and get confused when you can't find it.

The notebook is the animatedline. addpoints represents writing down the numbers. Closing the figure in which the animatedline "lives" is like throwing it in the shredder. When MATLAB gets confused it throws an error.

Calling isvalid is you specifically checking if the notebook exists (before trying to write down the number with addpoints.) Stopping the loop with break represents you giving up on writing down the numbers because you've learned the notebook no longer exists.

[Luke G.]

I see. So when you don't check if the notebook is still there, MATLAB freaks out because it wants to write (i.e., addpoints) in the notebook (i.e., animatedline) and not on the table. So this is basically saying "If the notebook is still on the table, write down the recorded data. Else, stop trying to write down data."

That's a great analogy, and I think I understand it now. Many thanks Steven!