You have to edit the function and provide an additional input to the function. The added input variable is the uiaxes that you want the plot to go on.
function plotfunction(arg1,arg2, plotaxes)
%.....
plot(plotaxes,arg1,arg2)
end
% then you can call it like this
function someappfunction(app)
% ....
plotfunction(X,Y,app.uiaxes);
end
