plot() creates line() objects. Each line object is drawn independently of the others, and only connects the points listed in the one line object. Also the default is not to put in any markers. Your a and b are scalars, so when you plot(a,b) they have no point to connect to so no line is drawn, and since the default is no marker, the individual points are not drawn. per's solution is to add a marker, so at least one point at a time would be drawn.
In order to have the points connected, you can:
- remember the previous point and draw back to it, so the graph would turn out to be made of a number of small lines; or
- store all of the points until the end of the loop and then plot all of the stored points at one time, so that a single line is drawn connecting all of them; or
- use animatedline() to create a basic line, and then each cycle of the loop, addpoints() to extend the line.