scatter plot with segment

[Yue Li]

I have a scatter plot like this:

p<-ggplot(df, aes(x=x,y=y, group = Year, colour=Year, shape =Year)) + geom_point()+ ...

The Year variable i use has four levels: Yr 1, 2, 3, 4. 

What i try to do is to show changes across years by adding arrow segments pointing from Yr 1 to Yr 2, from Y2 to Yr 3, and from Yr 3 to Yr 4. So i used something like this to connect points Yr1 and Yr2:

  geom_segment(aes(x=x1,y=y1,xend=x2,yend=y2), color = "grey20", arrow=arrow(length=unit(.1,"cm")))+

  geom_segment(aes(x=x2,y=y2,xend=x3,yend=y3), color = "grey20", arrow=arrow(length=unit(.1,"cm")))+

  geom_segment(aes(x=x3,y=y3,xend=x4,yend=y4), color = "grey20", arrow=arrow(length=unit(.1,"cm")))

The problem is that i have many missing points. So if a person only has data in Yr1 and Yr4, I still want to connect point Yr 1 with Yr 4, but don't know how. 

Is there a way to fix this? 

Many thanks!

Yue

[Hadley Wickham]

You will find this problem much much easier if you have tidy data:
http://vita.had.co.nz/papers/tidy-data.html

Hadley

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