Hello, Thank you for your reply. I am missing an intermediate step as
> plot( strptime( Time, format = "%H:%M:%S"), FreeMemory) Error in strptime(Time, format = "%H:%M:%S") : invalid `x' argument > plot( strptime( c(Time), format = "%H:%M:%S"), FreeMemory) Error in strptime(c(Time), format = "%H:%M:%S") : invalid `x' argument does not work. My "Time" data is part of a data.frame. Do I need to "as.POSIXlt" this in some way, and if so, how? I will continue to work on this, and buy a suitable book on R so as not to plague this excellent news group with such questions. I could generate a sequence of ISODates similar to my data, but I would like to use the actual Time coordinates instead. Here is my "Time" data below: > Time [1] 12:39:26 12:40:22 12:41:19 12:42:15 12:43:11 12:44:08 12:45:04 12:46:00 [9] 12:46:57 12:47:53 12:48:49 12:49:46 12:50:42 12:51:38 12:52:35 12:53:31 [17] 12:54:27 12:55:24 12:56:20 12:57:16 12:58:13 12:59:09 13:00:05 13:01:01 [25] 13:01:58 13:02:54 13:03:50 13:04:47 13:05:43 13:06:39 13:07:36 13:08:32 [33] 13:09:28 13:10:25 13:11:21 13:12:17 13:13:14 13:14:10 13:15:06 13:16:03 [41] 13:16:59 13:17:55 13:18:52 13:19:48 13:20:44 13:21:41 13:22:37 13:23:33 [49] 13:24:30 13:25:26 13:26:22 13:27:19 13:28:15 13:29:11 13:30:07 13:31:04 [57] 13:32:00 13:32:56 13:33:53 13:34:49 13:35:46 13:36:42 13:37:39 13:38:35 [65] 13:39:32 13:40:28 13:41:24 13:42:21 13:43:17 13:44:13 13:45:10 13:46:06 [73] 13:47:02 13:47:59 13:48:55 13:49:51 13:50:48 13:51:44 13:52:40 13:53:37 [81] 13:54:33 13:55:29 13:56:26 13:57:22 13:58:19 13:59:15 14:00:11 14:01:08 [89] 14:02:05 14:03:01 14:03:57 14:04:54 14:05:50 14:06:46 14:07:42 14:08:39 [97] 14:09:35 14:10:31 14:11:28 14:12:24 14:13:20 14:14:17 14:15:13 14:16:09 [105] 14:17:06 14:18:02 14:18:58 14:19:55 14:20:51 14:21:47 14:22:44 14:23:40 [113] 14:24:36 14:25:32 14:26:29 14:27:25 14:28:21 14:29:18 14:30:14 14:31:10 120 Levels: 12:39:26 12:40:22 12:41:19 12:42:15 12:43:11 12:44:08 ... 14:31:10 > Enjoying a little "R and R", John ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help