How to plot a sparse (with gaps) line with its segments colored according to some factor in R?

Question

I have a `data.frame` with time series. There are also `NA`s in it as well as there is a factor that I'd like to use to highlight different segments of a line. ``` flow.mndnr <- function(id, start, end) { uri <- sprintf("http://maps1.dnr.state.mn.us/cgi-bin/csg.pl?mode=dump_hydro_data_as_csv&site=%s&startdate=%s&enddate=%s", id, start, end) dat <- read.csv(url(uri), colClasses=c(Timestamp="Date")) rng <- range(dat$Timestamp) d <- data.frame(Timestamp=seq(rng[1], rng[2], by='day')) merge(d, dat, all.x=TRUE) } dat <- flow.mndnr("28062001", as.Date("2002-04-02"), as.Date("2011-10-05")) ``` I can plot it unconditionally ``` library(lattice) xyplot(Discharge..cfs. ~ Timestamp, dat, type='l', cex=0.5, auto.key=TRUE) ``` But I can't get rid of connecting lines when I try to introduce factor ``` xyplot(Discharge..cfs. ~ Timestamp, dat, type='l', groups=dat$Discharge..cfs..Quality, cex=0.5, auto.key=TRUE) ``` Same with ggplot2 ``` dat$quality <- dat$Discharge..cfs..Quality ggplot(dat, aes(x=Ti…

Accepted Answer

The problem is with the grouping. You can use the `year` to skip these jumps. Just do:

```
dat$grp <- format(dat$Timestamp, "%Y")
ggplot(dat, aes(x=Timestamp, y=Discharge..cfs.)) +
    geom_path(aes(colour = quality, group = grp)) + 
    theme(legend.position='bottom')
```

You get:

Edit: To answer the comment in detail: As long as you don't know which variable to group by, you can not group properly. If you have some months missing within the year, of course this code will produce jumps. In that case, I suggest doing something like this:

```
dat$grp <- paste(format(dat$Timestamp, "%Y"), format(dat$Timestamp, "%m"))
ggplot(dat, aes(x=Timestamp, y=Discharge..cfs.)) +
    geom_path(aes(colour = quality, group = grp)) + 
    theme(legend.position='bottom')
```

You get this:

How to plot a sparse (with gaps) line with its segments colored according to some factor in R?

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