How to use Greek symbols in ggplot2?

My categories need to be named with Greek letters. I am using ggplot2, and it works beautifully with the data. Unfortunately I cannot figure out how to put those greek symbols on the x axis (at the tick marks) and also make them appear in the legend. Is there any way to do it?

UPDATE: I had a look at the link, however, there is no good method described to accomplish what I want to do.

Here is a link to an excellent wiki that explains how to put greek symbols in ggplot2. In summary, here is what you do to obtain greek symbols

1. Text Labels: Use parse = T inside geom_text or annotate.
2. Axis Labels: Use expression(alpha) to get greek alpha.
3. Facet Labels: Use labeller = label_parsed inside facet.
4. Legend Labels: Use bquote(alpha == .(value)) in legend label.

You can see detailed usage of these options in the link

EDIT. The objective of using greek symbols along the tick marks can be achieved as follows

require(ggplot2);
data(tips);
p0 = qplot(sex, data = tips, geom = 'bar');
p1 = p0 + scale_x_discrete(labels = c('Female' = expression(alpha),
                                      'Male'   = expression(beta)));
print(p1);

For complete documentation on the various symbols that are available when doing this and how to use them, see ?plotmath.

Simplest solution: Use Unicode Characters

No expression or other packages needed.
Not sure if this is a newer feature for ggplot, but it works. It also makes it easy to mix Greek and regular text (like adding '*' to the ticks)

Just use unicode characters within the text string. seems to work well for all options I can think of. Edit: previously it did not work in facet labels. This has apparently been fixed at some point.

library(ggplot2)
ggplot(mtcars, 
       aes(mpg, disp, color=factor(gear))) + 
  geom_point() + 
  labs(title="Title (\u03b1 \u03a9)", # works fine
       x= "\u03b1 \u03a9 x-axis title",    # works fine
       y= "\u03b1 \u03a9 y-axis title",    # works fine
       color="\u03b1 \u03a9 Groups:") +  # works fine
  scale_x_continuous(breaks = seq(10, 35, 5), 
                     labels = paste0(seq(10, 35, 5), "\u03a9*")) + # works fine; to label the ticks
  ggrepel::geom_text_repel(aes(label = paste(rownames(mtcars), "\u03a9*")), size =3) + # works fine 
  facet_grid(~paste0(gear, " Gears \u03a9"))

^{Created on 2019-08-28 by the reprex package (v0.3.0)}

Use expression(delta) where 'delta' for lowercase δ and 'Delta' to get capital Δ.

Here's full list of Greek characters:

> Α α alpha
> Β β beta
> Γ γ gamma
> Δ δ delta
> Ε ε epsilon
> Ζ ζ zeta
> Η η eta
> Θ θ theta
> Ι ι iota
> Κ κ kappa
> Λ λ lambda
> Μ μ mu
> Ν ν nu
> Ξ ξ xi
> Ο ο omicron
> Π π pi
> Ρ ρ rho
> Σ σ sigma
> Τ τ tau
> Υ υ upsilon
> Φ φ phi
> Χ χ chi
> Ψ ψ psi
> Ω ω omega

EDIT: Copied from comments, when using in conjunction with other words use like: expression(Delta*"price")

You do not need the latex2exp package to do what you wanted to do. The following code would do the trick.

ggplot(smr, aes(Fuel.Rate, Eng.Speed.Ave., color=Eng.Speed.Max.)) + 
  geom_point() + 
  labs(title=expression("Fuel Efficiency"~(alpha*Omega)), 
color=expression(alpha*Omega), x=expression(Delta~price))

Also, some comments (unanswered as of this point) asked about putting an asterisk (*) after a Greek letter. expression(alpha~"*") works, so I suggest giving it a try.

More comments asked about getting Δ Price and I find the most straightforward way to achieve that is expression(Delta~price)). If you need to add something before the Greek letter, you can also do this: expression(Indicative~Delta~price) which gets you: