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This paper presents an analysis of the evolution of an open source software system, JFreeChart, which is an open source charting library , based on its size, fan-in/out coupling, and cohesion metrics. We developed JamTool, a Java automated measurement tool  to obtain the metrics and to observe the quality change along the evolution of the twenty-two released versions of JFreeChart. The empirical study clearly indicates that there are positive relations between the number of classes and the fan-in/out coupling, and the added class group has better software quality than the removed class group. Moreover, the experimental results follow Lehman's 1st, 2nd, 6th laws 6th but against 7th law of software evolution.