The emotion expressed by a music piece varies as the music unrolls in time. To create such dynamic expression, we develop an algorithm that automatically generates the accompaniment for a melody according to the emotion flow specified by a user. The emotion flow is given in the form of arousal and valence curves, each as a function of time. The affective accompaniment is composed of chord progression and accompaniment pattern. The chord progression, which controls the valence of the composed music, is generated by dynamic programming using the input melody and valence data as constraints. A mathematical model is developed to describe the temporal relationship between valence and chord progression. The accompaniment pattern, which controls the arousal of the composed music, is determined according to the quantized arousal values. The performance of the system is evaluated by subjective tests. The cross-correlation coefficient between the input arousal (valence) and the perceived arousal (valence) of the composed music is 0.85 (0.52). It rises to 0.92 for arousal and 0.88 for valence if only musician subjects in the test are considered. Overall, the proposed system is capable of generating subjectively appropriate accompaniments conforming to the user specification.