It is known that every planar graph has a planar embedding where edges are represented by non-crossing straight-line segments. We study the planar slope number, i.e., the minimum number of distinct edge-slopes in such a drawing of a planar graph with maximum degree A. We show that the planar slope number of every series-parallel graph of maximum degree three is three. We also show that the planar slope number of every planar partial 3-tree and also every plane partial 3-tree is at most 2~(O(Δ)). In particular, we answer the question of Dujmovic et al. [Computational Geometry 38 (3), pp. 194-212 (2007)] whether there is a function/such that plane maximal outerplanar graphs can be drawn using at most f(Δ) slopes.
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