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I am using ggplot2 to create a figure where the data plotted for different groups varies in both its linetype and color. I am trying to change the legend labels, however, each time I try to change the labels I am only able to change the labels for one aesthetic (i.e. color or linetype) and this produces two legends. Is there a way to change the legend labels when two aesthetics are mapped to the same variable?

Here is my working data:

# Generate working data
mydat <- structure(list(message_ineq_high_dum = c(0,0,1,1), 
 human_message_dum = c(1,0,1,0), 
 fit = c(65,60,76,66), 
 lwr = c(62,58,74,63), 
 upr = c(68,63,79,69), 
 var2 = structure(c(1L,1L, 2L, 2L), 
 .Label = c("Low", "High"),
 class = c("ordered","factor")), 
 var1 = structure(c(2L, 1L, 2L, 1L), 
 .Label = c("Low","High"), 
 class = c("ordered", "factor"))), 
 .Names = c("message_ineq_high_dum","human_message_dum", 
 "fit", "lwr", "upr", "var2",
 "var1"), 
 class = "data.frame", 
 row.names = c(NA, -4L)) 

Let's try to plot the data. The legend labels are "High" and "Low" in this plot.

# Plot: incorrect legend labels
library(ggplot2)

ggplot2::ggplot(data = mydat,
 aes(x = var2,
 y = fit,
 group = var1)) +
 theme_minimal() +
 geom_point(aes(color = var1)) +
 geom_line(aes(color = var1, linetype = var1)) +

 geom_errorbar(aes(ymin = lwr,
 ymax = upr,
 width = 0.1,
 color = var1, 
 linetype = var1)) +
 labs(x = "var2", y = "var1", color = "var3", linetype = "var3") +
 theme(panel.grid.minor = element_blank()) +
 scale_color_manual(values = c("Low" = "black", "High" = "red")) +
 theme(legend.position="bottom", legend.title = element_blank()) 

I am trying to change the legend labels from "High" and "Low" to "Foo" and "Bar" without changing anything else. Every approach I have tried changes the legend labels while also changing either (a) the number of legends and/or (b) the line type/color corresponding to the legend label.

You van use scale_linetype_manual, but don't forget to set the same labels!

library(ggplot2)

ggplot2::ggplot(data = mydat,
 aes(x = var2,
 y = fit,
 group = var1)) +
 theme_minimal() +
 geom_point(aes(color = var1)) +
 geom_line(aes(color = var1, linetype = var1)) +

 geom_errorbar(aes(ymin = lwr,
 ymax = upr,
 width = 0.1,
 color = var1, 
 linetype = var1)) +
 labs(x = "var2", y = "var1", color = "var3", linetype = "var3") +
 theme(panel.grid.minor = element_blank()) +
 scale_colour_manual(labels = c("Foo", "Bar"), values = c("black", "red")) +
 scale_linetype_manual( labels = c("Foo", "Bar"),values = c("solid", "dashed") ) +
 theme(legend.position="bottom", legend.title = element_blank())