How to connect points of different groups by a line using ggplot

Question

df<-data.frame(adjuster=c("Mary","Mary","Bob","Bob"), date=as.Date(c("2012-1-1","2012-2-1","2012-3-1","2012-4-1")), value=c(10,15,25,15))
df
  adjuster       date value
1     Mary 2012-01-01    10
2     Mary 2012-02-01    15
3      Bob 2012-03-01    25
4      Bob 2012-04-01    15

ggplot(df,aes(x=date,y=value,color=adjuster))+geom_line()+geom_point()

In the above graph, notice the disconnect between the February and March points. How do I connect those points with a blue line, leaving the actual March point red? In other words, Bob should be associated with the value from [Jan - Mar) and Mary from [Mar-Apr].

EDIT: Turns out my example was overly simple. The answers listed don't generalize to the case where the adjuster changes between two people on more than one occasion. For example, consider

df<-data.frame(adjuster=c("Mary","Mary","Bob","Bob","Mary"), date=as.Date(c("2012-1-1","2012-2-1","2012-3-1","2012-4-1","2012-5-1")), value=c(10,15,25,15,20))
      adjuster       date value
1     Mary 2012-01-01    10
2     Mary 2012-02-01    15
3      Bob 2012-03-01    25
4      Bob 2012-04-01    15
5     Mary 2012-05-01    20

Since I didn't mention this in my original question, I'll pick an answer that simply worked for my original data.

Answer 1

Updated to minimise tinkering with data.frame, added the group = 1 argument

Tinkered around with your data.frame a little. You should be able to automate the tinkering around, I guess. Let me know if you aren't. Also, your ggplot command wasn't working as per the chart you've posted in the question

df<-data.frame(
  adjuster=c("Mary","Mary","Bob","Bob"), 
  date=as.Date(c("2012-1-1","2012-2-1","2012-3-1","2012-4-1")), 
  value=c(10,15,25,15)
)

library(data.table)
library(ggplot2)
dt <- data.table(df)
dt[,adjuster := as.character(adjuster)]
dt[,prevadjuster := c(NA,head(adjuster,-1))]
dt[is.na(prevadjuster),prevadjuster := adjuster]


ggplot(dt) +
geom_line(aes(x=date,y=value, color = prevadjuster, group = 1)) +
geom_line(aes(x=date,y=value, color = adjuster, group = 1)) +
geom_point(aes(x=date,y=value, color = adjuster, group = 1))