[R] How to customize legend labels in ggplot2?

[Luigi Marongiu]

I have a data frame with measurements in different conditions. I set
the conditions as a factor using a notation for ease of use. I now
want to plot the data and assign meaningful labels to the factors. I
am using ggplot2; for the x axis I would like to keep the factors but
in the legend I would like to use custom values.
I tried different combinations but none worked.
What is the correct way to assign
custom labels to legends in ggplot2?
Thank you

EXAMPLE:
```
df = data.frame(Target = 1:4,
                Rate = c(0.02078663, 0.03685543, 0.02238002, 0.05033979),
                SD = c(0.003043398, 0.001447410, 0.002998729, 0.002171813))
df$Target = factor(df$Target)
ggplot(df, aes(x=Target, y=Rate, colour=Target, group=Target)) +
  geom_point(size=8) +
  geom_errorbar(aes(ymin=Rate-SD, ymax=Rate+SD), width=.1) +
  scale_colour_manual(values = COLS) +
  xlab(expression(bold("Class"))) +
  ylab(expression(bold("Value"))) +
  theme_classic(base_size = 15)
```
NOTE: if using
```
...
  theme_classic(base_size = 15, labels = c("Condition 1", "Condition 2",
                                           "Condition 3", "Control"))
```
I get the error:

Error in theme_classic(base_size = 15, labels = c("Condition 1",
"Condition 2",  :
  unused argument (labels = c("Condition 1", "Condition 2", "Condition
3", "Control"))