the actual melt command I am using:
DTODU <- melt(dtODU[,.SD,.SDcols=is.numeric],id.vars=c(1))
dput(head(dtODU))
structure(list(Smp = 12:17, Time = structure(c(1606221332, 1606221333,
1606221334, 1606221335, 1606221336, 1606221337), class = c("POSIXct",
"POSIXt"), tzone = "EST"), OMainMode = c("COOL", "COOL", "COOL",
"COOL", "COOL", "COOL"), RVState = c("COOL", "COOL", "COOL",
"COOL", "COOL", "COOL"), ODUFault = c("NO-FAULT", "NO-FAULT",
"NO-FAULT", "NO-FAULT", "NO-FAULT", "NO-FAULT"), LastFault = c("(31)OPER-CON-EXCEEDED",
"(31)OPER-CON-EXCEEDED", "(31)OPER-CON-EXCEEDED", "(31)OPER-CON-EXCEEDED",
"(31)OPER-CON-EXCEEDED", "(31)OPER-CON-EXCEEDED"), TargetFreq = c(75L,
75L, 75L, 75L, 75L, 75L), ActualFreq = c(75L, 75L, 75L, 75L,
75L, 75L), OAT = c(46L, 46L, 46L, 47L, 46L, 46L), HST = c(84L,
84L, 84L, 84L, 84L, 84L), OCT = c(54L, 54L, 54L, 54L, 54L, 54L
), CTT = c(86.2, 86.2, 86.2, 86.2, 86.2, 86.2), TCTT = c(85L,
85L, 85L, 85L, 85L, 85L), OMT = c(54L, 54L, 54L, 54L, 54L, 54L
), CompACCurr = c(0L, 0L, 0L, 0L, 0L, 0L), CompDCCurr = c(148L,
148L, 148L, 148L, 148L, 148L), SystemPower = c(0L, 0L, 0L, 0L,
0L, 0L), OFanSpd1 = c(85L, 86L, 86L, 86L, 85L, 85L), OFanSpd2 = c(86L,
86L, 86L, 85L, 85L, 85L), ODUMsg. = c(19L, 21L, 21L, 23L, 25L,
25L), TARG_EEV1 = c(332L, 332L, 332L, 332L, 332L, 332L), ACT_EEV1 = c(360L,
360L, 360L, 360L, 360L, 360L), RGLT1 = c(6L, 6L, 6L, 6L, 6L,
6L), TARG_EEV3 = c(396L, 396L, 396L, 396L, 396L, 396L), ACT_EEV3 = c(396L,
396L, 396L, 396L, 396L, 396L), RGLT3 = c(6L, 6L, 6L, 6L, 6L,
6L), TARG_EEV4 = c(378L, 378L, 378L, 378L, 378L, 378L), ACT_EEV4 = c(348L,
348L, 348L, 348L, 348L, 348L), RGLT4 = c(8L, 8L, 8L, 8L, 8L,
8L)), row.names = c(NA, -6L), class = c("data.table", "data.frame"
), .internal.selfref = <pointer: 0x000002245b4c1ef0>)