passing variable to 'gset arrow'

[David Larom]

I can't get octave to plot an arrow with variable instead of constant
input, while it works fine in gnuplot. In gnuplot, I can say:

plot(sin(x))
a=5
set arrow from a,0 to a,1
replot

and the arrow appears on the plot just fine.
HOWEVER,
if in octave I say:

b=-10:10;
plot(b,sin(b))
hold;
a=5
gset arrow from a,0 to a,1

I get the error message "line 0: undefined variable: a".
can anyone help? Is there a way to pass a variable to 
"gset arrow"?