var data = [{
    userId: 1, 
    year: 2016, 
    mailCount: 1234 
}, { 
    userId: 1, 
    year: 2017, 
    mailCount: 4321 
}, { 
    userId: 2,  
    year: 2016,  
    mailCount: 2345 }, {   userId: 2,   year: 2017,   mailCount 5678 }];