Add graphs produced by a for loop?

edit

Try this. Use p1=p1+ ... instead of p1 = ...

t=var('t')
for i in range(-3,3,1):
    p1 = p1+parametric_plot((t,i),(t,-5,5),color="red")
    p2 = p2+parametric_plot((i,t),(t,-5,5),color="gray")
show(p1+p2)

You'll also need to use Graphics() prior to your for loop to define p1 and p2 to be graphics objects.

t=var('t')
p1=Graphics()
p2=Graphics()
for i in range(-3,3,1):
    p1 +=parametric_plot((t,i),(t,-5,5),color="red")
    p2 +=parametric_plot((i,t),(t,-5,5),color="gray")
show(p1+p2)

Just on the off chance that you are trying to just make gridlines, here are some examples from the documentation:

sage: p = polar_plot(2 + 2*cos(x), 0, 2*pi, color=hue(0.3))
sage: p.show(gridlines=True,
...     hgridlinesstyle=dict(color="orange", linewidth=1.0),
...     vgridlinesstyle=dict(color="blue", linestyle=":"))

   Change the style of each grid line individually.

      sage: x, y = var('x, y')
      sage: p = implicit_plot((y^2-x^2)*(x-1)*(2*x-3)-4*(x^2+y^2-2*x)^2,
      ...             (x,-2,2), (y,-2,2), plot_points=1000)
      sage: p.show(gridlines=(
      ...    [
      ...     (1,{"color":"red","linestyle":":"}),
      ...     (0,{"color":"blue","linestyle":"--"})
      ...    ],
      ...    [
      ...     (-1,{"color":"red","linestyle":":"}),
      ...     (0,{"color":"blue","linestyle":"--"}),
      ...     (1,{"color":"red","linestyle":":"}),
      ...    ]
      ...    ),
      ...    gridlinesstyle=dict(marker='x',color="black"))

There are pretty finely-grained options available.