完整的jfreechart折线图应用示例。jfreechart版本1.0.19，可网上自行下载。Y轴为数值；X轴为字符型，通用性强。另外解决了生成的图片不被自动刷新的问题。-Complete jfreechart line chart application examples. jfreechart version 1.0.19, available online to download. Y-axis value X-axis for the character, versatility.