A novel wide-area control design is presented to mitigate inter-area power frequency oscillations. A large-scale power system is decomposed into a network of passivity-short subsystems whose nonlinear interconnections have a state-dependent affine form, and by utilizing the passivity shortage framework, a two-level design procedure is developed. At the lower level, any generator control can be viewed as one that makes the generator passivity-short and L2 stable, and the stability impact of the lower-level control on the overall system can be characterized in terms of two parameters. While the system is nonlinear, the impact parameters can be optimized by solving a data-driven matrix inequality (DMI), and the high-level wide-area control is then designed by solving another Lyapunov matrix inequality in terms of the design parameters. The proposed methodology makes the design modular, and the resulting control is adaptive to operating conditions of the power system. Standard test systems are used to illustrate the proposed design, including DMI and the wide-area control, and simulation results demonstrate its effectiveness in damping out inter-area oscillations.