Add blank lines for a couple plots in reference manual

[kcrisman]

In graphics.py, we have

plot(sin(pi*x), (x, -8, 8)) # Labels not so helpful
plot(sin(pi*x), (x, -8, 8), ticks=2) # Multiples of 2
plot(sin(pi*x), (x, -8, 8), ticks=[[-7,-3,0,3,7],[-1/2,0,1/2]]) # Your choices
plot(sin(pi*x), (x, -8, 8), ticks=[[],[]]) # No ticks at all!

but then in the live documentation only the last one shows up, so we should separate them with

::

things.

[kini]

Oh, is that why there are so many extra :: in Sage docstrings? I wondered about that.

Why not just wrap them with show() or put them in a single "line" by wrapping them with parentheses and semicolons?

show(plot(sin(pi*x), (x, -8, 8))) # Labels not so helpful
show(plot(sin(pi*x), (x, -8, 8), ticks=2)) # Multiples of 2
show(plot(sin(pi*x), (x, -8, 8), ticks=[[-7,-3,0,3,7],[-1/2,0,1/2]])) # Your choices
show(plot(sin(pi*x), (x, -8, 8), ticks=[[],[]])) # No ticks at all!

or

( plot(sin(pi*x), (x, -8, 8)) ; # Labels not so helpful
  plot(sin(pi*x), (x, -8, 8), ticks=2) ; # Multiples of 2
  plot(sin(pi*x), (x, -8, 8), ticks=[[-7,-3,0,3,7],[-1/2,0,1/2]]) ; # Your choices
  plot(sin(pi*x), (x, -8, 8), ticks=[[],[]]) ) # No ticks at all!

[kcrisman]

Well, whatever. We've basically been doing it this way for a while, and since plot(foo) does indeed implicitly call show, it's a little easier. There are some doctests out there that use the show() syntax. But I do prefer the :: business.

[kcrisman]

I don't see a problem with this. Positive review, thanks.