我画出了4个三维图像，现在想把它合并到一起，用gr combine说是这个三维图不是可编译的图像.....这有什么办法么还？？下面是源程序

clear
    set obs 900
    gen x = int((_n - mod(_n-1,30) -1 ) /30 )
    gen y = mod(_n-1,30)
    gen z1 = -0.491-0.116*x-0.133*y+0.16*x*y
    surface x y z1
    gr save mygraph1,replace
    clear
    set obs 900
    gen x = int((_n - mod(_n-1,30) -1 ) /30 )
    gen y = mod(_n-1,30)
    gen z2 = 0.491-0.102*x+0.369*y-0.544*x*y
    surface x y z2
    gr save mygraph2,replace
    clear
    set obs 900
    gen x = int((_n - mod(_n-1,30) -1 ) /30 )
    gen y = mod(_n-1,30)
    gen z3 = -0.491-0.186*x-0.133*y-0.077*x*y
    surface x y z3
    gr save mygraph3,replace
    clear
    set obs 900
    gen x = int((_n - mod(_n-1,30) -1 ) /30 )
    gen y = mod(_n-1,30)
    gen z4 = 0.491-0.074*x+0.369*y+0.599*x*y
    surface x y z4
    gr save mygraph4,replace
    gr combine mygraph1.gph mygraph2.gph mygraph3.gph mygraph4.gph,replace