Closely related to the safety and stability of power grids, stability analysis has long been a core topic in the electric industry. Conventional approaches employ computational simulation to make the quantitative judgement of the grid stability under distinctive conditions. The lack of in-depth data analysis tools has led to the difficulty in analytical tasks such as situation-aware analysis, instability reasoning and pattern recognition. To facilitate visual exploration and reasoning on the simulation data, we introduce WaveLines, a visual analysis approach which supports the supervisory control of multi-variate simulation time series of power grids. We design and implement an interactive system that supports a set of analytical tasks proposed by domain experts and experienced operators. Experiments have been conducted with domain experts to illustrate the usability and effectiveness of WaveLines.