Given a positive integer $d$, the class $d$-DIR is defined as all those intersection graphs formed from a finite collection of line segments in ${\mathbb R}^2$ having at most $d$ slopes. Since each slope induces an interval graph, it easily follows for every $G$ in $d$-DIR with clique number at most $ω$ that the chromatic number $χ(G)$ of $G$ is at most $dω$. We show for every even value of $ω$ how to construct a graph in $d$-DIR that meets this bound exactly. This partially confirms a conjecture of Bhattacharya, Dvořák and Noorizadeh. Furthermore, we show that the $χ$-binding function of $d$-DIR is $ω\mapsto dω$ for $ω$ even and $ω\mapsto d(ω-1)+1$ for $ω$ odd. This extends an earlier result by Kostochka and Nešetřil, which treated the special case $d=2$.