How to create trendlines that are partly solid and partly dashed?

Question

I am working on a time series data analysis, for which I created a plot. In addition to showing the data points, I want to display a trend line. For this, the pre-intervention trend is supposed to be a solid line, whereas the "counterfactual scenario" (post-intervention trend line) is supposed to be dashed. So what I need is a line that is solid until 2005 and dashed from then on.

So far, I just managed to create a continuously solid line. This is what my plot looks like so far (I'm afraid I don't know how to use syntax hihlighting):

Q=ts(c(7.5,6.9,6.2,6.4,7.4,7.2,6.9,6.5,8.1,12.3,10.6,11.7,12.2,13.07,12.48,14.54,15.06,15.1,13.46,13.1,13.75,11.81,13.33,15.75), start = 1995, end =2018, frequency = 1)

tQ<-c(1995,1996,1997,1998,1999,2000,2001,2002,2003,2004,2005,2006,2007,2008,2009,2010,2011,2012,2013,2014,2015,2016,2017,2018)

plot(Q,type = "n", main = "Numbers per Year", xlab = "Year", ylab = "Q Number", ylim = c(0,35), xaxt='n')

rect(2005,-1,2018.5,36,col=grey(0.9),border=F) #post-intervention period

points(AR)

axis(side=1,at=c(1995,1996,1997,1998,1999,2000,2001,2002,2003,2004,2005,2006,2007,2008,2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018) , las = 2)

abline(v = 2005, lty = 2)

text(Q, labels = Q, cex = 0.7, pos = 3, offset = 1)

abline(lm(Q~tQ))

So far I haven't found any unseful information how to change the trendline.

Thank you very much for your help!

Answer 1

I am not so sure if this is what you are looking for but. Those codes give a dashed line after 2005. You just need to cancel out the very last abline() in your codes and add those lines below,

 #abline(lm(Q~tQ))
 x <- tQ[11:length(tQ)]
 y <- Q[11:length(Q)]
 lines(x,fitted(lm(y~x)),col="blue",lty = 2)

Or, if you want to add a trend line starts to be dashed until 2005,

#abline(lm(Q~tQ))
fit <- lm(Q~tQ)
lines(tQ[1:11],fitted(fit)[1:11],col="blue")
lines(tQ[11:length(tQ)],fitted(fit)[11:length(tQ)],col="blue",lty = 2)

And lastly, if you desire to get a separated trend lines by before and after 2005,

 #abline(lm(Q~tQ))
 x1 <- tQ[1:11]
 y1 <- Q[1:11]
 x2 <- tQ[11:length(tQ)]
 y2 <- Q[11:length(Q)]
 lines(x1,fitted(lm(y1~x1)),col="blue")
 lines(x2,fitted(lm(y2~x2)),col="blue",lty = 2)