[R] problem with the behaviour of dashed lines in R plots

Eric Berger ericjberger at gmail.com
Mon Dec 4 20:49:30 CET 2017

Sarah's last comment about using min/max x got me thinking. It's not that
the points are "very close together", it's that the x-values are not
ordered. So the plot is actually drawing a dashed line back-and-forth
between different points on the line, which has the effect of making the
result appear non-dashed. If you sort by the x-values before plotting you
will see the output is very different. I am not saying this is a better
solution than Sarah's regarding just using the end-points, but at least it
partially explains the relevance of that suggestion.

For example, here is a slightly modified version of the code that does an
ordering before plotting:

regressor<-lm(A~B,data = df1)
df2 <- data.frame( x=df1$B, yhat= predict(regressor,df1))
df2 <- df2[ order(df2$x), ]
plot(df2$x,df2$yhat,type="l", col="black", mgp=c(2,0.5,0),cex.lab=1.6,
lwd=2, lty=2,xlim=range(c(1.2,1.7)),ylim=rev(range(c(-19,-8))))
par(new = TRUE)
plot(df1$B,as.numeric(df1$A),type="p", col="black",
mgp=c(2,0.5,0),cex.lab=1.6,cex=2, xlab = "", ylab =


On Mon, Dec 4, 2017 at 8:30 PM, jean-philippe <
jeanphilippe.fontaine at gssi.infn.it> wrote:

> hi Sarah,
> Thanks a lot for having taken time to answer me and for your reply. I
> wonder how I missed this solution. Indeed plotting the line with the 2
> extreme data points works perfectly.
> Best,
> Jean-Philippe Fontaine
> On 04/12/2017 18:30, Sarah Goslee wrote:
>> It's because you are plotting a line between each of the points in
>> your data frame, and they are very close togethe
> --
> Jean-Philippe Fontaine
> PhD Student in Astroparticle Physics,
> Gran Sasso Science Institute (GSSI),
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> 67100 L'Aquila, Italy
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