为什么在画共同趋势分析图的时候，首尾的点会连在一起？附上代码：
egen mean=mean( ln市场价格), by(diff_week treat)
twoway (connect mean diff_week if treat==1& diff_week<=4 & diff_week>=-4,lpattern(longdash)lwidth(thick)) (connect mean diff_week if treat==0 & diff_week<=4 & diff_week>=-4,lwidth(thick)) if diff_week<=4 & diff_week>=-4,xline(0,lpattern(longdash) lwidth(medium thick)) title("农产品价格平均增长趋势") ytitle("农产品价格") xtitle("距离春节周数")  legend(label(1 "实验组") label(2 "控制组")) xscale(range (-4  4)) xlabel(-4 (1) 4)