AI Chat Playground generated an interactive plot with a slider and a bug

I asked MATLAB AI Chat Playground to generate an interactive plot with a slider.

With slight modification, the code runs (in desktop environment) , but the plot is updated only when the slider value

is increased, not when decreased. Can anyone spot why?

% Create the UI figure
fig = uifigure('Name', 'Interactive Sin Plot');
fig.Position(3:4) = [400 300];

% Create the plot
ax = uiaxes(fig);
ax.Position = [50 50 300 150];
x = linspace(0, 2*pi, 1000);
y = sin(x);
line(ax, x, y, 'Color', 'b', 'LineWidth', 2);

% Create the slider
slider = uislider(fig);
slider.Position = [50 200 300 3];
slider.Limits = [0.1 10];
slider.Value = 1;
slider.MajorTicks = [0.1 1 10];
slider.ValueChangedFcn = @(sld, event) updatePlot(sld.Value, ax);
% Function to update the plot based on the slider value
function updatePlot(period, ax)
x = linspace(0, 2*pi*period, 1000);
y = sin(x);
line(ax, x, y, 'Color', 'b', 'LineWidth', 2);
end

Answer 1

[Ayush Modi]

Hi Tomazz,

This seems like a bug. As a workaround, use plot function instead of line function. This updated the plot even when the slider value is decreased.

plot(ax, x, y, 'Color', 'b', 'LineWidth', 2);

Please refer to the following MathWorks documentation for more information on the plot function:

https://www.mathworks.com/help/matlab/ref/plot.html

As for why line function is not working, you can report it to the Technical Support team by creating a Service Request: https://mathworks.com/support/contact_us.html

[Tomazz]

Hi Ayus,

if I wrote the function, I would use plot function, too.

It seems this is a wierd behavior of line function.

Thanks,

Tomazz

Answer 2

[DGM]

Try just updating the existing line object instead of creating new line objects.

% Create the UI figure
fig = uifigure('Name', 'Interactive Sin Plot');
fig.Position(3:4) = [400 300];

% Create the plot
ax = uiaxes(fig);
ax.Position = [50 50 300 150];
x = linspace(0, 2*pi, 1000);
y = sin(x);
hl = line(ax, x, y, 'Color', 'b', 'LineWidth', 2);

% Create the slider
slider = uislider(fig);
slider.Position = [50 200 300 3];
slider.Limits = [0.1 10];
slider.Value = 1;
slider.MajorTicks = [0.1 1 10];
slider.ValueChangedFcn = @(sld, event) updatePlot(sld.Value, hl);

% Function to update the plot based on the slider value
function updatePlot(period, hl)
    x = linspace(0, 2*pi*period, 1000);
    y = sin(x);
    hl.XData = x;
    hl.YData = y;
end

[Tomazz]

Thank you,

this solution works, and you're right it's better to update data than to add another line object

in every updatePlot call.

However, I would still like to understand the asymmetry in behavior, when I'm sliding up or down.

[DGM]

The reason is because the axes extents are set implicitly by the range of the data plotted by all the line objects, and the old line objects don't get deleted. As a consequence, the axes limits only change if you create a set of XData which spans a larger interval than any prior plot. The new "invisible" lines are there, they're just coincident with the existing lines over a shorter interval.

You can see the behavior in action if you use a different line color for each plot.

line(ax, x, y,'color',rand(1,3),'LineWidth', 2);

FWIW, chart.line primitives created by plot() have a few different interactions with their parent axes upon creation when compared to graphics.line primitives created by line(). They're similar, but just bear in mind they're not really expected to be the same.

[Tomazz]

Cool, all clear now, thanks!