ext() objects are included in legend() .
You need to record the handles returned by plot(). Then you call legend() and as the first parameter to legend() you pass the handles of the graphics objects you are creating legend entries for.
When you plot(a,b,'*') you are generating a single line() object with three markers that are all the same.
line() objects and scatter() objects have the same difficulty that each line and each scatter plot can only have a single marker shape and color assigned to it. If you need different marker shapes or colors for your three a, b points then you will need to make three calls to plot() or scatter()
