This issue has been fixed in MATLAB R2023b
Starting in MATLAB R2023b, provide the axis handle as the first input to plot the fit line in your app's axes.
plot(app.UIAxes, fitline, a, b)
Workaround prior to R2023b: create plot externally, then copy to AppDesigner axes
Prior to R2023b, plotting a cfit object is not supported with uifigures (e.g. AppDesigner; see Graphics Support in App Designer). Even if it were supported, for whatever reason there is no way to specify an axis handle when plotting a cfit object, although that problem isn't difficult to solve.
For more info see
Workaround: You can plot the fit object in an external, temprary figure, copy its content to the App's axes, and then delete the temporary figure.
% Create uifigure and uiaxes to mimic AppDesigner
app.UIFigure = uifigure();
app.UIAxes = uiaxes(app.UIFigure);
% Plot cfit object externally.
a = linspace(0,10,10)';
b = linspace(0,20,10)';
[fitline,gof] = fit(a, b, 'poly1');
fig = figure('Visible','off'); % you may want to set this to "on" to see what's happening
ax = axes(fig);
h = plot(fitline,a,b);
% Copy content of temp axes to app designer
hApp = copyobj(h, app.UIAxes);
% You'll need to recreate the legend since it can't be independently copied
lh = legend(app.UIAxes);
% Delete temp figure
delete(fig)
