按照PPT给的命令和数据我怎么一个图也画不出来，怎么回事，请高手帮忙
.
. histogram  mpg
(bin=8, start=12, width=3.625)
object Graph (not a graph) exists:
      Graph cannot be used as a graph name r(110);
twoway scatter mpg weight
object Graph (not a graph) exists:
      Graph cannot be used as a graph name r(110);
. use "E:\新建文件夹\GC\Stata统计分析与应用视屏教程\Stata统计分析与应用视屏教程\d
> ata\教程数据\chap03\consumption_china.dta", clear
. twoway line c y year
object Graph (not a graph) exists:
      Graph cannot be used as a graph name r(110);