How do I get the plot in my for loop to retain previous plots?

I'm trying to set up a for loop inside a function to plot my data so I can see it evolving. Here's the relevant code. My function updates 'T'; z is just a vector of the same length.

% Plot every hundredth iteration; this is so I can see if it's changing. 
    c = c + 1; 
    if c == 100
        c = 0; 
        fig = figure(1);
        axh = axes;
        plot(axh,T,z)
        hold all
        grid on
        set(gca,'YDir','Reverse')
        title('Evolving Geotherm')
        xlabel('Temperature (K)'); ylabel('Depth (m)')
    end

This will display ONLY the latest data; none of the previous plots remain. I get the same results when I use 'hold on' and 'drawnow'. This seems like a really simple bit of code.

What's going on here? Why won't the previous plots remain?

Answer 1

[Chris]

You are regenerating the figure and axes every time the if condition triggers. About the only thing you need inside the if is the plot() command. The other stuff should go before the for loop (but inside the function).

fig = figure(1);
axh = axes
hold all
grid on
set(gca,'YDir','Reverse')
title('Evolving Geotherm')
xlabel('Temperature (K)'); ylabel('Depth (m)')

c = 0;
for idx = 1:whatever

    % Do some loop stuff...
    
    c = c+1;
    if c == 100
        c = 0;
        plot(axh,T,z)
    end

end

[Joshua Knicely]

Having just the plot command in the for loop was my first instinct, but it doesn't work. If I don't recreate the figure and axes inside the if (and for) loop, then the plot doesn't appear until after the function has run its course, making the entire plot pointless.

[Chris]

You can try adding drawnow inside the loop:

    if c == 100
        c = 0;
        plot(axh,T,z)
        drawnow
    end

[Jeff Miller]

I agree with Chris, but maybe you also need 'drawnow':

    if c == 100
        c = 0;
        plot(axh,T,z)
        drawnow;
    end

[Chris]

The above applies if you call the function once and the for loop is what updates the data.

If you are calling the whole function to recalculate your data, then there's a function scope problem which would be a slightly more involved fix.

[Joshua Knicely]

@Chris & @Jeff Miller, y'all were correct about the drawnow. With that, it shows my plot as the data changes.

@Chris, not sure about meaning with the function scope. Would that be like an accidental recursive call of the function on itself? My data is updated in the for loop.

[Chris]

I was searching for other reasons it could be failing. If the function were calculating one point each time it's called, over repeated calls, it might not have access to all points to pass to plot. I was stretching my imagination a bit, since I couldn't see how the function was working.