Hello,
You could use dput(), it's not your first post...
last <- structure(list(date = c("8/6/2008", "8/8/2008", "8/11/2008",
"8/13/2008", "8/14/2008", "8/18/2008", "8/20/2008", "8/27/2008",
"8/28/2008", "8/31/2008", "9/2/2008", "9/3/2008", "9/4/2008",
"9/5/2008", "9/8/2008", "9/11/2008", "10/12/2009", "10/14/2009",
"10/19/2009", "10/21/2009", "10/26/2009", "10/29/2009", "10/30/2009",
"11/2/2009", "11/3/2009", "11/4/2009", "11/11/2009", "11/13/2009"
), r_wvht = c(0.9766667, 0.7733333, 1.4833333, 1.5766667, 1.39,
0.78, 0.8383333, 1.77, 1.295, 2.41, 1.3166667, 1.3075, 1.39,
1.6333333, 1.2416667, 1.395, 0.8633333, 2.79, 1.0325, 1.965,
1.78, 1.5666667, 1.05, 1.4633333, 1.24, 1.0075, 1.605, 1.8475
)), .Names = c("date", "r_wvht"), class = "data.frame", row.names = c("1",
"2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13",
"14", "15", "16", "17", "18", "19", "20", "21", "22", "23", "24",
"25", "26", "27", "28"))
last$date <- as.Date(last$date, format="%m/%d/%Y")
plot(r_wvht ~ date, data = last)
(You were forgetting that last$date was a character vector, not class Date.)
Hope this helps,
Rui Barradas