Try this:
plot(app.UIAxes_2,x,lket,'b')
Generally arguments to plot come in x/y pairs. That is, you specify the x values and then the corresponding y values for each line you want to plot. If you only specify a single vector, then that's treated as y, and it is plotted against the indices 1:numel(y).
So you are correct: the reason you don't get any negative x values is because you've given plot one vector to plot. Give it two vectors and you'll get a plot with negative x (if there are negative values in the x you give it, of course), as above.
