Consistent color scale and legend between plots when not all levels of a grouping variable are present in the data

Set the factor levels of Colours to include all possible values, whether present or not in the data at hand, then add drop=FALSE to scale_colour_manual:

all.data=data.frame(Date=dates15, Data=ex.data, Colours=colours)
g.cols=c("black", "blue", "pink", "purple")
all.data$Colours = factor(all.data$Colours, levels=sort(c(unique(colours), "Pink Points")))

ggplot(all.data, aes(Date, Data, colour=Colours, group=1)) + 
  geom_line() +
  scale_color_manual(values=g.cols, drop=FALSE) +
  xlim(as.POSIXct("2015-01-01 00:00:00"), as.POSIXct("2015-02-12 23:45:00"))

Consistent color scale and legend between plots when not all levels of a grouping variable are present in the data

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