' 
' Red diagonal Line
'
SCREEN 12
COLOR 4                  ' red
LINE (0, 0) - (639, 497) ' line from upper left to lower right
END

Several Lines

The image will look like this (in this picture, the size of the graphics screen has been made much smaller and the line has been made thicker):

Here is a program that draws a small square in the upper left corner:

' 
' Small Red Square
'
SCREEN 12
COLOR 4
LINE (1, 1) - (1, 10)   ' Left side of square
LINE (10, 1) - (10, 10) ' Right side of square
LINE (1, 1) - (10, 1)   ' Top side of square
LINE (1, 10) - (10, 10) ' Bottom of square

END

On graph paper, the square looks like this:

Of course, on the actual graphics screen the red square is much smaller.