stata中用twoway line命令分别画出三条折线图，现在需要叠加在一张图里。graph save后用graph combine只能得到三张图分布在一起，没有重叠。请问应该用什么命令？

twoway line idealpoint year if countryname == "United States of America", yscale(range(-3 3)) ylabel(-3(1.5)3)
graph save russiaidealpoint.gph
绘制Russia折线图并保存
twoway line idealpoint year if countryname == "Russia", yscale(range(-3 3)) ylabel(-3(1.5)3)
graph save usidealpoint.gph
绘制USA折线图并保存

twoway line MedianIdealpoint year, yscale(range(-3 3)) ylabel(-3(1.5)3)
graph save MedianIdealpoint.gph
绘制所有国家的每年的理想点中位数折线图并保存
graph combine russiaidealpoint.gph usidealpoint.gph MedianIdealpoint.gph