Guide to LaTeX (4th Edition) (Tools and Techniques

288 Chapter 13. Drawing with L AT E X
The coordinate numbers are generally positive, meaning that the point
lies to the right and above the reference point. Since this point is the
lower left corner of the picture, all other points should be more to the
right and higher than it. However, negative values are also possible. A
negative x value (a negative number for the first member of a coordinate
pair) defines a point left of the origin, while a negative y value (the second
member of the pair is negative) specifies a point below the reference point.
y-axis
✻
1.4
origin
✠
✛ UL ✲
(1.8,1.4)
✲ x-axis
1.8
(−2.2,1.8)
negative coordinate values
y
✻
(−1.8,−0.8)
The unit length is selected with the command
\setlength{\unitlength}{length}
✛UL✲ (3.0,2.0)
(2.5,−1.0)
✲ x
In the left-hand example above, the unit length was set to a value of
1.5 cm with the command \setlength{\unitlength}{1.5cm}. The
point (1.8,1.4) then lies 1.8 times the unit length (= 2.7 cm) to the right,
and 1.4 times (= 2.1 cm) above the origin. In the right-hand example, the
unit length is set to 1 cm.
The unit of length is normally set to a convenient size such as 1 cm,
1 mm, or 1 in and the picture is built up accordingly. Once it has been
completed, it is possible to rescale the whole thing simply by changing
the value of the length unit. A picture that was originally designed with
\unitlength set to 1 cm can be enlarged by a factor 1.2 by redefining its
length unit to be 1.2 cm.
13.1.2 The picture environment
Pictures are constructed within the picture environment which is started
with
\begin{picture}(x dimen,y dimen)
picture commands
\end{picture}