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You can create a list of points indicating the periods. (I use list comprehension. )

f(x)=-2*sin(4*(x+pi/4))
P=points([(k*pi/2,0) for k in [-4..4]], pointsize=40, color='red')
P+plot(f(x),(x,-2*pi,2*pi), ticks=pi/2, tick_formatter=pi)