Problem shading area between two vertical lines

[David Beheshti]

I'm using Stata/IC 14.2.

I want to shade the area between two vertical lines, but have two problems:

1. The shading does not cover the entire vertical range of the plot. I believe I could fix this if there were a way to identify the maximum and minimum y values that Stata includes in graph region, but I have been unable to figure out how to do this. Is there a way to know before plotting what these values are? I am making several figures in a loop so eyeballing it won't help.
2. The vertical lines are covered by the shading. Is there a way to fix this?

Below is an example which illustrates what I am trying to do:

clear
sysuse sp500

gen t = _n

twoway (scatteri -50 70  -50 80, bcolor(gs12) recast(area) plotr(m(zero))) ///
       (scatteri 90 70  90 80, bcolor(gs12) recast(area) plotr(m(zero))) ///
       (scatter change t), ///
       xline(70 80, lcolor(black) lpattern(dash)) ///
       legend(off)

This produces the following figure:

If I could replace the values of -50 and 90 with the edges of the plot area I believe that would solve #1. I'm not quite sure about #2.