Scale color error when trying to put a frame in a range with geom_rect over a geom

First I will simulate the data I am working with for replication.

  electrode <- c('Cz','Fz','Pz')
  value <- c(2.307785,1.613350,2.925053)
  sd <- c(0.7593193,0.7612393,0.7568794)
  se <- c(0.002829816,0.002836971,0.002820723)
  ci <- c(0.005546431,0.005560455,0.005528609)
  tgc <- data.frame(electrode,value,sd,se,ci)

  col <- c("#004d8d", "#cc2701", "#e5b400")
  stdev <- c(10.783939,10.506721,9.415955)

See I also specify the color and then use a manual scale in the ggplot.

Now if I put the geom_errorbefore the geom_rect that adds the shade I want, it works fine but it does layer the shade over the points with error bars, as you can see with the Fz in the middle.

  p1 <- ggplot(tgc, aes(x=factor(electrode), y=value, colour=electrode))  
    geom_errorbar(aes(ymin=value-sd, ymax=value sd), width=.1, alpha=.5)  
    geom_rect(inherit.aes = FALSE,mapping =aes(xmin=0, xmax=Inf, ymin=-0.1*min(stdev), ymax=0.1*max(stdev)),fill="grey",alpha = 0.5)  
    scale_color_manual(values = col, breaks = c("Fz", "Cz", "Pz"))     
    scale_fill_manual(values = col, breaks = c("Fz", "Cz", "Pz"))  
    labs(title="Difference at max. discrepancy", x="Electrode", y="Signal (µV)")  
    lims(y = c(-10, 10))  
    theme_linedraw()  
    geom_point()

  p1

What I am trying to do is change the layer order, and to do so, I interchange places of geom_rect with geom_errorbar.

  p1 <- ggplot(tgc, aes(x=factor(electrode), y=value, colour=electrode))  
    geom_rect(inherit.aes = FALSE,mapping =aes(xmin=0, xmax=Inf, ymin=-0.1*min(stdev), ymax=0.1*max(stdev)),fill="grey",alpha = 0.5)  
    geom_errorbar(aes(ymin=value-sd, ymax=value sd), width=.1, alpha=.5)  
    scale_color_manual(values = col, breaks = c("Fz", "Cz", "Pz"))     
    scale_fill_manual(values = col, breaks = c("Fz", "Cz", "Pz"))  
    labs(title="Difference at max. discrepancy", x="Electrode", y="Signal (µV)")  
    lims(y = c(-10, 10))  
    theme_linedraw()  
    geom_point()

  p1

And I get the following error message and it does not draw the new plot.

Error: Discrete value supplied to continuous scale

And I don't really understand why this is happening as I am, in fact, specifying fill in geom_rect.

Any ideas what I am doing wrong? Thanks in advance!

CodePudding user response：

The issue is that by switching the order a continuous x scale is "added" to your plot via the geom_rect which when adding the geom_errorbar raises an error as there you are mapping a categorical variable on x. In contrast, doing it the other way around is fine, i.e. we could map a continuous value on an otherwise discrete scale.

A fix would be to explicitly set a discrete x scale by adding a scale_x_discrete before the geom_rect:

library(ggplot2)

ggplot(tgc, aes(x = factor(electrode), y = value, colour = electrode))  
  scale_x_discrete()  
  geom_rect(inherit.aes = FALSE, mapping = aes(xmin = 0, xmax = Inf, ymin = -0.1 * min(stdev), ymax = 0.1 * max(stdev)), fill = "grey", alpha = 0.5)  
  geom_errorbar(aes(ymin = value - sd, ymax = value   sd), width = .1, alpha = .5)  
  scale_color_manual(values = col, breaks = c("Fz", "Cz", "Pz"))  
  scale_fill_manual(values = col, breaks = c("Fz", "Cz", "Pz"))  
  labs(title = "Difference at max. discrepancy", x = "Electrode", y = "Signal (µV)")  
  lims(y = c(-10, 10))  
  theme_linedraw()  
  geom_point()