想画一种逐年变化图
比如2008-2010年共三年，有两列数据，日期   温度
                                    1       20
                                    2       21
                                    3       20
                                    .        .
                                    .        .
                                   365      24
                                    1       20
                                    2       21
                                    3       20
                                    .        .
                                    .        .
                                   365      24
                                    1       20
                                    2       21
                                    3       20
                                    .        .
                                    .        .
                                   365      24
    如果直接画图plot（x，y），横坐标就成了1-365，把三年的数据就叠加画到一起了，我想把每年的日变化一年接一年的画出来，怎么画？