\begin{axis}[
     xlabel=x in $m^2$,
     ylabel=y in $m^2$,
     tick align=inside,
     y tick label style={/pgf/number format/.cd,%
         set thousands separator={}},%取消数字中的“，” 比如2,000变成2000
     width=3in,
     height=3in,
     legend pos=outer north east,
     legend style={}
     ]
 \addplot[smooth,mark=*,black] plot coordinates {
     (1999,1000)
     (2000,2000)
     (2001,3000)
     (2002,4000)
     (2003,5000)
     (2004,6000)
 };
 \addlegendentry{line}