df <- structure(list(ID = structure(c(1L, 1L, 2L, 2L, 3L, 3L, 4L, 4L,
5L, 5L, 6L, 6L), .Label = c("1", "2", "3", "4", "5", "6"), class = "factor"),
    cluster = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 3L,
    3L, 3L, 3L), .Label = c("1", "2", "3"), class = "factor"),
    val = c(1.2581800436601, 6.79055672604591, 9.77732860250399,
    3.60806297743693, 1.14399523707107, 7.9990872181952, 3.16242988454178,
    5.64627967076376, 8.82345798192546, 4.29119206266478, 8.62997844815254,
    6.46683012368158), date = c(1, 2, 1, 2, 1, 2, 1, 2, 1, 2,
    1, 2)), class = "data.frame", row.names = c(NA, -12L))

   ID cluster      val date
1   1       1 1.258180    1
2   1       1 6.790557    2
3   2       1 9.777329    1
4   2       1 3.608063    2
5   3       2 1.143995    1
6   3       2 7.999087    2
7   4       2 3.162430    1
8   4       2 5.646280    2
9   5       3 8.823458    1
10  5       3 4.291192    2
11  6       3 8.629978    1
12  6       3 6.466830    2