Lines - $m, k \in \mathbb{R}$:

<UL>
   <LI>Decreasing or horisontal, $m \leq 0$: $y=mx+k$.</LI>
   <LI>Vertical: $x=k$.</LI>
   <LI>Increasing, $m > 0$:
      \[
          y(x)=
          \left\{
             \begin{matrix}
                mx+b,&x \leq 0
                \\
                \frac{1}{2}m x+b,&x> 0
             \end{matrix}
          \right.
      \]
   </LI>
</UL>