The imported data are quite a mess. I did some manual revisions. Try this code.
MarriageData2 <- structure(list(X1 = c("Response:","Breakdown:", " ", "Subjective class identification", "Lower class","Working class"),
X2 = c("Strongly agree", "Subjective class identification","Year", "1994", "\r\n14.3\r\n\r\n(4.47)\r\n", "\r\n11.9\r\n\r\n(1.41)\r\n"),
X3 = c(NA, NA, NA, "1998", "\r\n23.5\r\n\r\n(5.38)\r\n", "\r\n17.0\r\n\r\n(1.78)\r\n"),
X4 = c(NA, NA, NA, "2002", "\r\n19.1\r\n\r\n(5.63)\r\n", "\r\n20.3\r\n\r\n(1.77)\r\n")), row.names = c(NA, 6L), class = "data.frame")
and then run
View(MarriageData2)
Does that look like what you would expect? If so, what do you want to graph? I know that this is only the first six lines of the data