Linkaxes, auto scale and debugger unusual behaviour

I am using R2023a. When I run the code below, I get a different result if I set a breakpoint. If I run the code straight through, the y axis limits end up ad just within plus/minus 25. If I set a breakpoint at the linkaxes statement and then step through the code, the y axis limits end up at plus/minus 5.

Further, I am not sure how this is supposed to behave, but the behaviour I want is for all 3 plots to auto scale to whatever plot has the highest amplitudes. When I step through the code with the debugger, each of the auto scale lines at the end of the code rescales all 3 plots to a lower scale, so the first plot goes off the chart.

Code originally from here: https://au.mathworks.com/help/matlab/ref/linkaxes.html

h = tiledlayout(3, 1);

plotList = [];

% First plot
h1 = nexttile(h);
hold on
plotList = [plotList; h1];
x1 = linspace(0,6);
y1 = 8*sin(x1);
line1 = plot(h1, x1,y1);

% Second plot
h2 = nexttile(h);
hold on
plotList = [plotList; h2];
x2 = linspace(0,10);
y2 = 2*sin(2*x2);
plot(h2, x2,y2)

% Third plot
h3 = nexttile(h);
hold on
plotList = [plotList; h3];
x3 = linspace(0,12,200);
y3 = 4*sin(6*x3);
plot(h3, x3,y3)

linkaxes(plotList, 'xy')
ylim(h1, 'auto')
ylim(h2, 'auto')
ylim(h3, 'auto')

set(line1, 'YData', 3*y1)

Answer 1

[Dinesh]

Hello.

The discrepancy in the y-axis limits you are observing as I understand it is likely because the "ylim('auto')" command is executed before the update to the "YData" property of "line1", which changes the range of data displayed. To resolve this, you may move the "set(line1, 'YData', 3*y1)" command to before the "ylim('auto')" commands. This ensures that when MATLAB calculates the automatic y-axis limits, it considers the updated data range.

The correct behavior which is having all plots auto-scale to the plot with the highest amplitude will be achieved by setting the "YData" first, then linking the axes with "linkaxes(plotList, 'xy')", and finally using "ylim('auto')" to auto-scale the y-axis according to the updated data. This way, running the code either with or without breakpoints will consistently give you the same result, with all three plots scaled appropriately.

Documentation for "ylim": https://www.mathworks.com/help/matlab/ref/ylim.html

[CM]

Hi Dinesh, for the situation I want to use the code in, I want to link the axes once, set the limits to automatic once, then update the plotted data values multiple times, and have all plots automatically rescale when I send a drawnow command. Is that possible?

I don't think your explanation makes sense. If I run the code without the debugger, the axes limits are updated as you would expect from set(line1, 'YData', 3*y1). Also, if I comment out the last line and run the code without debugging it, the plots are scaled to plus or minus 2, which must be coming from the 2nd of 3 plots. The result seems to be fairly random when I run the code at full speed. It is only when I step through the code with the debugger that the results are predictable, though not what I want.

[Dinesh]

My understanding was incorrect. I thought your query was that the y-values printed by setting a breakpoint differ from what you see in the figure. Now that I read it, the figure itself seems to be different when you run the code directly versus using a debugger and then clicking "Continue" after the breakpoint is hit.

I could also reproduce the issue that you were facing. If I find a solution for this, I will update my answer accordingly.

[CM]

Thanks Dinesh!