If the user changed something so that your plotted data changed, (like the x or y values) then you need to call the function (plot, bar, pie, plotyy, whatever) again to update it with the new data.
If the appearance of the plot (like the title, xlabel, size, whatever) was supposed to change, then it should change right away.
The only time when replotting or changing appearance won't work right away is if you're in some intensive loop that's going so fast and furious that the "behind the scenes" message to update/refresh your plot (or image or axes) never gets acted upon. In that situation you can call " drawnow" inside your loop to force it to update on each loop iteration.
