plot sqrt(sin(x))

If you only want to get rid of the part where sin(x) is negative you can use

maxima.plot2d(sqrt(sin(x)),[x,0,10])

(write this in sage - it has a maxima interface built in).

However, since the values of sqrt(sin(x)) in these places are complex, a possibly better visualization of it is

complex_plot(sqrt(sin(x)), (0, 10), (-1, 1))

This is a strip around the range you are interested in. Here is an explanation of the output, from sage manual:

The magnitude of the output is indicated by the brightness (with zero being black and infinity being white) while the argument is represented by the hue (with red being positive real, and increasing through orange, yellow, ... as the argument increases).

This page demonstrates and explains some complex plots.

I think that the current best way to deal with your issue is to know ahead of time what the domain is.

sage: plot(sqrt(sin(x)),(x,0,pi))+plot(sqrt(sin(x)),(x,2*pi,3*pi))

Which isn't ideal, but could be enough for you.

However, I agree that the current behavior looks pretty bad, so Ticket 11123 is now dedicated to this.

As another workaround, you could do

sage: plot(sqrt(sin(x)),(0,10),plot_points=40000)

but even that looks not the greatest, and takes forever.