sysuse auto,clear
gen time=_n
gen control=0.02*time+6
gen treat_1=0.03*time+9
gen treat_2=0.08*time+7
*然后求两条线的交点 time=40 y=10.2
gen treat=treat_1
replace treat=treat_2 if treat_2>treat_1
tset time
tsline control  treat