access.id | data
    1        0
    1        0
    2       150
   ...      ...

access.id | V1 | V2
    1       0    0
    2      150  ...

test[, lapply(.SD, .fun=function(dt) { return (transpose(dt)) }) , by = access.id]

test[, lapply(.SD, .fun=function(dt) { return (transpose(dt[data])) }) , by = access.id]

test[, .SD[, transpose(data)] , by = access.id]

test[, lapply(.SD, .fun=function(dt) { return (transpose(dt$data)) }) , by = access.id]

test[, .SD[, t(data)] , by = access.id]