Allow plotting zero width interval

[kcrisman]

plot(x,x,0,0)
       	
Traceback (click to the left of this block for traceback)
...
ValueError: plot start point and end point must be different

Maybe this shouldn't be an error, but instead give an empty plot or a point? See below for discussion.

[kini]

See #11753, which should have fixed this.

[kini]

Indeed I cannot reproduce this on Sage 5.0.beta13.

[kcrisman]

Oh, and this is clearly a dup and invalid as it stands, because I was using our old campus server. Nice!

But I'm going to change this. I wonder if this maybe shouldn't give rise to an empty graphic, or maybe just a point. The reason is that in the wild, one can't always predict what one will want. As an example:

var('y')
@interact
def _(t = [pi/6*n for n in [1..18]]):
    P = plot(1/2*sin(y),(x,0,t),fill=True,tick_formatter=pi,ticks=pi/6)
    Q = plot(1/2*sin(y),(x,0,3*pi),tick_formatter=pi,ticks=pi/6)
    show(P+Q)
    html("The integral up to $y=%s$ is $\\approx %s$."%(latex(t),latex(integral(1/2*sin(y),y,0,t).n())))

I would really like to have it be from [0..18], since I already have a plot and it would be nice to have this work since integration from 0 to 0 works - but this error stands in the way.