各位大神，我想知道，我用stata做平行趋势检验，下面这张图的虚线为什么在前一期呢，不应该在当期吗，我用的代码如下：

forvalues i =4(-1)1{
gen pre_`i' = (period == -`i' & treat ==1)
}
*将政策实施的年份（1994）与treat做交互
gen current = (period == 0 & treat ==1)
*将政策实施后的年份与treat做交互
forvalues j =1(1)4{
gen time_`j' = (period == `j'&treat ==1)
}
*进行固定效用回归

xtreg lngini time treat pre_4 pre_3 pre_2 pre_1    current time_*  lntrade lnu lnenroll rule perGDP gii_  i.year,fe
est sto reg
coefplot reg,keep(pre_* current time_*) vertical recast(connect) yline(0) xline(4,lp(dash)) ytitle("Effect of BRT") xtitle("time （pre_*befofe，current initiative，time_*after）")

是我用的代码不对吗