The IEEE 802.15.4e MAC amendment has been proposed to meet the requirements of industrial applications. Using slotted medium access with channel hopping, the MAC layer orchestrates the medium accesses of nodes according to a given schedule. Nevertheless, this amendment does not specify how this schedule is computed. The purpose of this paper is to propose a distributed joint time slot and channel assignment, called Wave, for data gathering in low-power lossy networks. This schedule targets minimised data convergecast delays by reducing the total number of slots in the schedule. Moreover, Wave ensures the absence of conflicting transmissions in the schedule provided. In such a schedule, a node is awake only during its transmission slots and those of its children in the convergecast routing graph. Thus, energy efficiency is ensured. In this paper, we present Wave with its properties (e.g. support of heterogeneous traffic, support of a sink equipped with multiple interfaces, worst case delays and buffer size) and compare it with a centralised scheduling algorithm like TMCP and a distributed one like DeTAS. Simulation results show the good performance of Wave compared with TMCP. Because in an industrial environment, several routing graphs can coexist, we study how Wave supports this coexistence.