One of Atari Portfolio's weak points is the graphics. Not only has Portfolio very modest graphics capabilites (what can be understood), but also is hardly compatible with PCs. I think this is why there were so few games written for Pofo (Folidash and Phoenix are two of them).
There are two ways to display graphics on Portfolio: using interrupt 10h or sending data directly to the graphics controller. The former is very slow, so it's only intended to use when you want to introduce simple graphic elements (e.g. in board games), but its advantage is the compatibility - you'll be able to run your program on a PC. The latter is much faster - it's actually the only way to program animations - but your code will hang the PC.
Using int 10h is also much easier. If you want to draw an oblique line, you may type:
mov ax, 4 ; set graphics mode 4 int 10h mov cx, 0 ; initial X coordinate mov dx, 0 ; initial Y loop: mov ax,0c01h ; set pixel mov bh, 0 int 10h cmp cx, 63 je end inc cx inc dx jmp loop end: mov ah, 1 ; wait for a keypress int 21h mov ax, 7 ; set graphics mode 7 int 10h int 20h ; exit to DOS