Continuous X-Axis plot in Real Time

I am plotting some data from arduino in Matlab in real-time. The problem is that I am unable to get the x-axis (samples) to increase in samples without starting from 0 again. So I have the data with an interval of 200 samples (interv = 200;) and the axis are plotted using: axis([0,interv,0,3]). All this is under a while(1) loop so it plots the graph continuously but once the samples reach 200, its starts to plot again from 0 and the process is repeated. So what I would like to do is once 200 is reached then continue the samples to display from 200-400 and then 400-600 and so on. I don't want to use auto-axis since it will just squash the data.

Answer 1

[Geoff Hayes]

bilal - please see https://www.mathworks.com/matlabcentral/answers/284202-how-to-plot-a-real-time-signal-with-axes-automatically-updating which has an example that is similar to yours. It proposes that a timer be used to update the axes (rather than a while loop) and updates the data for the graphics object that was used to plot the data

 xdata = [get(handles.hPlot,'XData') handles.T(end)];
 ydata = [get(handles.hPlot,'YData') handles.array(end)];    
 set(handles.hPlot,'XData',xdata,'YData',data);

where

 handles.hPlot = plot(NaN,NaN); % creates the graphics object with no data

Or, if you just want to set the limits on the x-axis, you could probably use xlim to set the limits on the x-axis given the current set of samples

 numSamples = 200;
 atBlock = 1;
 xlim([(atBlock-1)*numSamples atBlock*numSamples]);

[bilal malik]

Thanks for your response Geoff, I think that example is a lot more complex than what I am trying to do. I have added my code below.

I don't want to set a limit on the x-axis but I just simply want it to plot continuously on the x-axis so from 0 - 200 samples, then from 200-400, 400-600, 600-800 and so on until the code is aborted.

clc
clear all
a = arduino('COM4', 'Due');
while(1)
    interv = 200;
    k = 1;
    t=1;
    x=0;
    while(t<interv)
        b=readVoltage(a, 'A0');
        x=[x,b];
        plot(x);
        xlabel('Samples')
        ylabel('Voltage')
        title('Phonocardiogram')
        axis([0,interv,0,3]);
        grid
        t=t+k;
        drawnow;
        pause(0.002)
   end
end

[Geoff Hayes]

bilal - ok, the problem may be with your initialization of x

 x = 0;

which is reset on each iteration of the outer while loop...which would explain why the data starts to plot again from 0. I would either initialize it to be an empty array as

 x = [];

or after the end of the inner while loop (so once it completes) set it to be the last element of x

 x = 0;
 while(1)
     % etc.
     while t<interv
         % etc.
     end
     x = x(end);
 end

In the second example, x is initialized to zero before we start the outer while loop and re-initialize it once the inner while loop completes.

But either of the above might not be sufficient because you still set the axis limits as

 axis([0,interv,0,3]);

and interv never changes from 200 so you will always be fixing the x axis limits to [0, 200]. If you want to shift the interval by 200 on each iteration of the inner while loop, you may have to do something like

 clc
 clear all
 a = arduino('COM4', 'Due');
 y=0;
 hPlot = plot(NaN);
 intervalSize = 200;
 currentInterval = 200;
 t = 1;
 atInterval = 1;
 while(1)
    k = 1;
    while(t<currentInterval)
        b=readVoltage(a, 'A0');
        y=[y,b];
        set(hPlot, 'YData', y);
        xlabel('Samples')
        ylabel('Voltage')
        title('Phonocardiogram')
        axis([currentInterval - intervalSize,currentInterval,0,3]);
        grid
        t=t+k;
        pause(0.002)
    end
   currentInterval = currentInterval + intervalSize;
   atInterval = atInterval + 1;
 end

In the above, I've changed some of the variable names to distinguish between the interval size (200) and the current interval. I've also replaced x with y since the voltage (which is what you are reading) is the variable along the y-axis whereas the number of samples is along the x-axis.

Note the

 axis([currentInterval - intervalSize,currentInterval,0,3]);

which ensures that the x-axis limits are always (currentInterval-200, currentInterval) which would be (0,200), (200,400), etc.

Each time that you call plot, you create a new graphics object. Given that you have two while loops, this means that you are creating several hundred objects when all you really need is one that you update again and again. I've simplified this to a single graphics object which you can re-use on each iteration of the loop.

There are probably other improvements that you can make to the code (one being that all samples are kept even those from intervals that are no longer being shown because of the limits to the axes, so you can exclude all of this old data if no longer needed).

[bilal malik]

Thank you very much Geoff. This is exactly what I was after. :)

[bilal malik]

Hi Geoff - on the above code it works fine but when I close the graph, the following displays in the command window:

Error using matlab.graphics.chart.primitive.Line/set
Invalid or deleted object.

Error in newPCG (line 24)
        set(hPlot, 'YData', y);

Do you have any ideas as to why that my be? Thanks

[Geoff Hayes]

Hi bilal - you are closing the figure so the handle hPlot (of the graphics object drawn on the figure) is no longer valid. You're while loop doesn't know this and so throws an error when it tries to call

 set(hPlot, 'YData', y);

since hPlot is no longer valid. I suppose you could put a check around this as

 if ishandle(hPlot)
     set(hPlot, 'YData', y);
 else
     break;  % break out of the loop
 end

You would then need another check in the outer while loop to decide whether you should break out of that one too

 while(1)
    k = 1;
    while(t<currentInterval)
        b=readVoltage(a, 'A0');
        y=[y,b];
        if ishandle(hPlot)
            set(hPlot, 'YData', y);
        else
            break;  % break out of the loop
        end
        xlabel('Samples')
        ylabel('Voltage')
        title('Phonocardiogram')
        axis([currentInterval - intervalSize,currentInterval,0,3]);
        grid
        t=t+k;
        pause(0.002)
    end
   currentInterval = currentInterval + intervalSize;
   atInterval = atInterval + 1;
   if ~ishandle(hPlot)
       break;
   end
 end 
 % close connection to arduino

[bilal malik]

That makes sense and has solved the problem. Thank you :)