Connecting across missing values with geom_line

Question

I'm trying to figure out if it's possible to connect across missing values using geom_line. For example, in the link below there are missing values at time 3 in facet F. I'd like a line to connect time 2 and 4 in that case. Is there a way to achieve this?

https://farm8.staticflickr.com/7061/6964089563_b150e0c2a6.jpg

I have a data frame of cumulative values like so:

head(cumulative)

  individual series Time     Value
1          A      x    1 -1.008821
2          A      x    2 -2.273712
3          A      x    3 -3.430610
4          A      x    4 -4.618860
5          A      x    5 -4.893075
6          A      x    6 -5.836532

Which I'm plotting with:

ggplot(cumulative, aes(x=Time,y=Value, shape=series)) + 
    geom_point() + 
    geom_line(aes(linetype=series)) + 
    facet_wrap(~ individual, ncol=3)

Answer 1

Lines aren't drawn if a value is NA. You need to replace these by interpolating across missing points. There are many different algorithms for interpolation, you need to experiment with several and see which one suits your data best. This example uses linear interpolation via interp1 in the pracma package.

Sample data:

dfr <- data.frame(
  x = 1:10,
  y = runif(10)
)
dfr[c(3, 6, 7), "y"] <- NA

Interpolation step:

dfr$z <- with(dfr, interp1(x, y, x, "linear"))

Compare plots:

ggplot(dfr, aes(x, y)) + geom_line()
ggplot(dfr, aes(x, z)) + geom_line()

If you are showing this graph to other people, make sure that you clearly mark the places where you've synthesised data by interpolating (maybe using dotted lines).

---

Update based on comment:
You can specify different aesthetics for different geoms.

ggplot(dfr, aes(x)) + 
  geom_point(aes(y = y)) +
  geom_line(aes(y = z))

To incorporate different line types for missing/non-missing y, you can do something like

ggplot(dfr, aes(x)) + 
  geom_point(aes(y = y)) +
  geom_line(aes(y = y)) +
  geom_line(aes(y = z), linetype = "dotted")

Answer 2

Richie's answer is very thorough, but I wanted to show something simpler. Since lines are not drawn to NA points, another approach is drop these points when drawing lines. This implicitly makes a linear interpolation between points (as straight lines do).

Using dfr from Richie's answer, without needing the calculation of z step:

ggplot(dfr, aes(x,y)) + 
  geom_point() +
  geom_line(data=dfr[!is.na(dfr$y),])

For that matter, in this case the subsetting could be done for the whole thing.

ggplot(dfr[!is.na(dfr$y),], aes(x,y)) + 
  geom_point() +
  geom_line()