Dynamically create expression for ggplot legend?

I want to plot a line graph with multiple lines colored based on a grouping variable. Now I want to set legend labels using the scale

-command:

scale_color_manual(values = colors_values, labels = ...)

      

        
        
        
      

    

The legend characters are "x ^ 2", "x ^ 3", "x ^ 4", etc., where the range is dynamically created. Now I would like to dynamically create an expression as a label text i.e.

• "x^2"
  
  should become x ²
• "x^3"
  
  should become x ³

and etc.

The number of legend labels changes, so I thought of something like as.expression(sprintf("x^%i", number))

that, of course, doesn't work as a parameter label

to a function scale

.

I've searched google and Stack Overflow, however I haven't found a working solution yet, so hope someone can help me here.

Here's a reproducible example:

poly.term <- runif(100, 1, 60)
resp <- rnorm(100, 40, 5)
poly.degree <- 2:4
geom.colors <- scales::brewer_pal(palette = "Set1")(length(poly.degree))
plot.df <- data.frame()
for (i in poly.degree) {
  mydat <- na.omit(data.frame(x = poly.term, y = resp))
  fit <- lm(mydat$y ~ poly(mydat$x, i, raw = TRUE))
  plot.df <- rbind(plot.df, cbind(mydat, predict(fit), sprintf("x^%i", i)))
}
colnames(plot.df) <- c("x","y", "pred", "grp")
ggplot(plot.df, aes(x, y, colour = grp)) +
  stat_smooth(method = "loess", se = F) +
  geom_line(aes(y = pred))
  scale_color_manual(values = geom.colors
                     # here I want to change legend labels
                     # lables = expresion???
                     )

      

        
        
        
      

    

I would like the legend labels to be x ² x ³ and x ⁴ .