Greek letters, symbols, and line breaks inside a ggplot legend label

Baptiste's suggestion was spot on: unicode go a long way. Replace every instance of gamma above with \u03B3 and it will display as Greek. Amazing!

Printing can be done with the cairo device (ref):

library(Cairo)
cairo_pdf(file = "ggplot-greek.pdf", width = 8, height = 5)
## ggplot object created here
dev.off()

Also works: ggsave("greek.pdf", device = cairo_pdf)

Here is a selection of unicode symbols I have found useful:

"\U016B" = ū
"\U016A" = Ū
"\U00FB" = û
"\U00DB" = Û
"\U0233" = ȳ
"\U0232" = Ȳ
"\U0177" = ŷ
"\U0176" = Ŷ
"\U0113" = ē
"\U0112" = Ē
"\u00EA" = ê
"\U00CA" = Ê
"\U003BC\U2080" = μ₀
"\U003BC\U2081" = μ₁
"R\U00B2" = R²
"\u221E" = ∞
"\u2248" = ≈
"\U2260" = ≠ 
"\u03C3/\u221An" = σ/√n

Sadly, not all special characters have a unicode equivalent. For instance, there is a unicode for y-hat but not for x-hat. It is possible to combine the letter x with a unicode bar, as in "x\u0305" = x̅ or "p\u0302" = p̂, but that doesn't seem to print very well.

I have also had difficulty printing greek unicode as axis labels, so in these cases I resorted to annotate() with the option coord_cartesian(clip = "off") to ensure that the labels printed below the axes do not get clipped ("on" is the default clipping behaviour).

Sometimes you can directly use the character, for instance:

myplot + labs(title  = "±")

When unicode fails, you can try expression(), e.g. x̄:

title = expression(paste("z = (", bar(x), "-\u03BC)/(\u03C3/\u221An)", sep = ""))  
library(ggplot2)
ggplot() + labs(title = title) + theme_bw() 

For convenience, I copy some commonly used greek letters' unicodes below. Many more unicodes may be found, e.g. http://en.wikipedia.org/wiki/List_of_Unicode_characters

\u0391  Α   Greek Capital Letter Alpha
\u0392  Β   Greek Capital Letter Beta
\u0393  Γ   Greek Capital Letter Gamma
\u0394  Δ   Greek Capital Letter Delta
\u0395  Ε   Greek Capital Letter Epsilon
\u0396  Ζ   Greek Capital Letter Zeta
\u0397  Η   Greek Capital Letter Eta
\u0398  Θ   Greek Capital Letter Theta
\u0399  Ι   Greek Capital Letter Iota
\u039A  Κ   Greek Capital Letter Kappa
\u039B  Λ   Greek Capital Letter Lambda
\u039C  Μ   Greek Capital Letter Mu
\u039D  Ν   Greek Capital Letter Nu
\u039E  Ξ   Greek Capital Letter Xi
\u039F  Ο   Greek Capital Letter Omicron
\u03A0  Π   Greek Capital Letter Pi
\u03A1  Ρ   Greek Capital Letter Rho
\u03A3  Σ   Greek Capital Letter Sigma
\u03A4  Τ   Greek Capital Letter Tau
\u03A5  Υ   Greek Capital Letter Upsilon
\u03A6  Φ   Greek Capital Letter Phi
\u03A7  Χ   Greek Capital Letter Chi
\u03A8  Ψ   Greek Capital Letter Psi
\u03A9  Ω   Greek Capital Letter Omega
\u03B1  α   Greek Small Letter alpha
\u03B2  β   Greek Small Letter beta
\u03B3  γ   Greek Small Letter gamma
\u03B4  δ   Greek Small Letter delta
\u03B5  ε   Greek Small Letter epsilon
\u03B6  ζ   Greek Small Letter zeta
\u03B7  η   Greek Small Letter eta
\u03B8  θ   Greek Small Letter theta
\u03B9  ι   Greek Small Letter iota
\u03BA  κ   Greek Small Letter kappa
\u03BB  λ   Greek Small Letter lambda
\u03BC  μ   Greek Small Letter mu
\u03BD  ν   Greek Small Letter nu
\u03BE  ξ   Greek Small Letter xi
\u03BF  ο   Greek Small Letter omicron
\u03C0  π   Greek Small Letter pi
\u03C1  ρ   Greek Small Letter rho
\u03C2  ς   Greek Small Letter final sigma
\u03C3  σ   Greek Small Letter sigma
\u03C4  τ   Greek Small Letter tau
\u03C5  υ   Greek Small Letter upsilon
\u03C6  φ   Greek Small Letter phi
\u03C7  χ   Greek Small Letter chi
\u03C8  ψ   Greek Small Letter psi
\u03C9  ω   Greek Small Letter omega

Information about which fonts to use with unicode: https://en.wikipedia.org/wiki/List_of_typefaces#Unicode_fonts

Some currency symbols:

# http://www.fileformat.info/info/unicode/category/Sc/list.htm
Character   Name    Browser     Image
\u0024  DOLLAR SIGN     $
\u00A2  CENT SIGN   ¢
\u00A3  POUND SIGN  £
\u00A4  CURRENCY SIGN   ¤
\u00A5  YEN SIGN    ¥
\u058F  ARMENIAN DRAM SIGN
\u060B  AFGHANI SIGN    ؋
\u09F2  BENGALI RUPEE MARK  ৲
\u09F3  BENGALI RUPEE SIGN  ৳
\u09FB  BENGALI GANDA MARK
\u0AF1  GUJARATI RUPEE SIGN     ૱
\u0BF9  TAMIL RUPEE SIGN    ௹
\u0E3F  THAI CURRENCY SYMBOL BAHT   ฿
\u17DB  KHMER CURRENCY SYMBOL RIEL  ៛
\u20A0  EURO-CURRENCY SIGN  ₠
\u20A1  COLON SIGN  ₡
\u20A2  CRUZEIRO SIGN   ₢
\u20A3  FRENCH FRANC SIGN   ₣
\u20A4  LIRA SIGN   ₤
\u20A5  MILL SIGN   ₥
\u20A6  NAIRA SIGN  ₦
\u20A7  PESETA SIGN     ₧
\u20A8  RUPEE SIGN  ₨
\u20A9  WON SIGN    ₩
\u20AA  NEW SHEQEL SIGN     ₪
\u20AB  DONG SIGN   ₫
\u20AC  EURO SIGN   €
\u20AD  KIP SIGN    ₭
\u20AE  TUGRIK SIGN     ₮
\u20AF  DRACHMA SIGN    ₯
\u20B0  GERMAN PENNY SIGN   ₰
\u20B1  PESO SIGN   ₱
\u20B2  GUARANI SIGN    ₲
\u20B3  AUSTRAL SIGN    ₳
\u20B4  HRYVNIA SIGN    ₴
\u20B5  CEDI SIGN   ₵
\u20B6  LIVRE TOURNOIS SIGN     ₶
\u20B7  SPESMILO SIGN   ₷
\u20B8  TENGE SIGN  ₸
\u20B9  INDIAN RUPEE SIGN   ₹
\u20BA  TURKISH LIRA SIGN   ₺
\u20BB  NORDIC MARK SIGN    ₻
\u20BC  MANAT SIGN  ₼
\u20BD  RUBLE SIGN  ₽
\uA838  NORTH INDIC RUPEE MARK
\uFDFC  RIAL SIGN   ﷼
\uFE69  SMALL DOLLAR SIGN   ﹩
\uFF04  FULLWIDTH DOLLAR SIGN   ＄
\uFFE0  FULLWIDTH CENT SIGN     ￠
\uFFE1  FULLWIDTH POUND SIGN    ￡
\uFFE5  FULLWIDTH YEN SIGN  ￥
\uFFE6  FULLWIDTH WON SIGN