I am using R to plot some data.
Date <- c(\"07/12/2012 05:00:00\", \"07/12/2012 06:00:00\", \"07/12/2012 07:00:00\",
\"07/12/2012 08:00:00\",\"07/
Juba's answer, to include explicit NA's where you want breaks, is the best approach. Here is an alternate way to introduce those NA's in the right place (without having to figure it out manually).
every.hour <- data.frame(Date=seq(min(Date), max(Date), by="1 hour"))
df2 <- merge(df1, every.hour, all=TRUE)
g %+% df2

You can do something similar with your later df example, after changing the dates and times into a proper format
df$DateTime <- as.POSIXct(strptime(paste(df$Date, df$Time),
format="%m/%d/%Y %H:%M:%S"))
every.ten.seconds <- data.frame(DateTime=seq(min(df$DateTime),
max(df$DateTime), by="10 sec"))
df.10 <- merge(df, every.ten.seconds, all=TRUE)