Data type of dropdown.Value is a character, so you need to covert it to number.
The following will work. Please replace "plot" command depending on your data.
function startupFcn(app)
plot(app.UIAxes, 1:20, 1:20)
app.xl = xline(app.UIAxes, str2double(app.DCValueDropDown.Value));
end
function DCValueDropDownValueChanged(app, event)
value = app.DCValueDropDown.Value;
% Change xline value
app.xl.Value = str2double(value);
end
Below is a preview.
