R語言繪圖時輸出希臘字符上下標及數學公式實現方法
通常在我們寫論文時，所需要的統計圖是非常嚴謹的，裡面的希臘字符與上下腳標都必須要嚴格書寫。因此在使用R繪圖時，如何在我們目標圖中使用希臘字符、上標、下標及一些數學公式呢？在本博客中我們會進行詳細的說明。
後面我們都將以一個最簡單的繪圖為例，隻是將其標題進行修改。
希臘字母
使用希臘字符、上標、下標及數學公式，都需要利用一個函數：expression()，具體使用方式如下：
plot(cars) title(main = expression(Sigma))
輸出：
上下標
expression()中的下標為[]，上標為^，空格為~，連接符為*。示例代碼如下：
plot(cars) title(main = expression(Sigma[1]~'a'*'n'*'d'~Sigma^2))
輸出：
paste
想達到上面的效果，我們其實可以使用paste()與expression()進行組合，不需要上述繁瑣的過程，也能夠達到我們上述一模一樣的輸出，並且方便快捷：
plot(cars) title(main = expression(paste(Sigma[1], ' and ', Sigma^2)))
一個復雜的例子
目標：
代碼：
expression(paste((frac(1, m)+frac(1, n))^-1, ABCD[paste(m, ',', n)]))
進階
在我們想批量產生大量含有不同變量值的標題時，如果遇到變量與公式的混合輸出該如何操作，
可參考前文：R語言繪圖公式與變量對象混合拼接實現方法
數學公式
最後的數學公式，隻需要在expression()中進行相應的符號連接即可
具體要求可參考：Mathematical Annotation in R
鑒於其很不穩定，這裡將裡面的細節搬運過來。
（下表也可以直接在 R help 中搜索 plotmath 獲取。）
| Syntax | Meaning |
|---|---|
| x + y | x plus y |
| x – y | x minus y |
| x*y | juxtapose x and y |
| x/y | x forwardslash y |
| x %±% y | x plus or minus y |
| x %/% y | x divided by y |
| x %*% y | x times y |
| x %.% y | x cdot y |
| x[i] | x subscript i |
| x^2 | x superscript 2 |
| paste(x, y, z) | juxtapose x, y, and z |
| sqrt(x) | square root of x |
| sqrt(x, y) | yth root of x |
| x == y | x equals y |
| x != y | x is not equal to y |
| x < y | x is less than y |
| x <= y | x is less than or equal to y |
| x > y | x is greater than y |
| x >= y | x is greater than or equal to y |
| !x | not x |
| x %~~% y | x is approximately equal to y |
| x %=~% y | x and y are congruent |
| x %==% y | x is defined as y |
| x %prop% y | x is proportional to y |
| x %~% y | x is distributed as y |
| plain(x) | draw x in normal font |
| bold(x) | draw x in bold font |
| italic(x) | draw x in italic font |
| bolditalic(x) | draw x in bolditalic font |
| symbol(x) | draw x in symbol font |
| list(x, y, z) | comma-separated list |
| … | ellipsis (height varies) |
| cdots | ellipsis (vertically centred) |
| ldots | ellipsis (at baseline) |
| x %subset% y | x is a proper subset of y |
| x %subseteq% y | x is a subset of y |
| x %notsubset% y | x is not a subset of y |
| x %supset% y | x is a proper superset of y |
| x %supseteq% y | x is a superset of y |
| x %in% y | x is an element of y |
| x %notin% y | x is not an element of y |
| hat(x) | x with a circumflex |
| tilde(x) | x with a tilde |
| dot(x) | x with a dot |
| ring(x) | x with a ring |
| bar(xy) | xy with bar |
| widehat(xy) | xy with a wide circumflex |
| widetilde(xy) | xy with a wide tilde |
| x %<->% y | x double-arrow y |
| x %->% y | x right-arrow y |
| x %<-% y | x left-arrow y |
| x %up% y | x up-arrow y |
| x %down% y | x down-arrow y |
| x %<=>% y | x is equivalent to y |
| x %=>% y | x implies y |
| x %<=% y | y implies x |
| x %dblup% y | x double-up-arrow y |
| x %dbldown% y | x double-down-arrow y |
| alpha – omega | Greek symbols |
| Alpha – Omega | uppercase Greek symbols |
| theta1, phi1, sigma1, omega1 | cursive Greek symbols |
| Upsilon1 | capital upsilon with hook |
| aleph | first letter of Hebrew alphabet |
| infinity | infinity symbol |
| partialdiff | partial differential symbol |
| nabla | nabla, gradient symbol |
| 32*degree | 32 degrees |
| 60*minute | 60 minutes of angle |
| 30*second | 30 seconds of angle |
| displaystyle(x) | draw x in normal size (extra spacing) |
| textstyle(x) | draw x in normal size |
| scriptstyle(x) | draw x in small size |
| scriptscriptstyle(x) | draw x in very small size |
| underline(x) | draw x underlined |
| x ~~ y | put extra space between x and y |
| x + phantom(0) + y | leave gap for “0”, but don’t draw it |
| x + over(1, phantom(0)) | leave vertical gap for “0” (don’t draw) |
| frac(x, y) | x over y |
| over(x, y) | x over y |
| atop(x, y) | x over y (no horizontal bar) |
| sum(x[i], i==1, n) | sum x[i] for i equals 1 to n |
| prod(plain§(X==x), x) | product of P(X=x) for all values of x |
| integral(f(x)*dx, a, b) | definite integral of f(x) wrt x |
| union(A[i], i==1, n) | union of A[i] for i equals 1 to n |
| intersect(A[i], i==1, n) | intersection of A[i] |
| lim(f(x), x %->% 0) | limit of f(x) as x tends to 0 |
| min(g(x), x > 0) | minimum of g(x) for x greater than 0 |
| inf(S) | infimum of S |
| sup(S) | supremum of S |
| x^y + z | normal operator precedence |
| x^(y + z) | visible grouping of operands |
| x^{y + z} | invisible grouping of operands |
| group(“(“,list(a, b),”]”) | specify left and right delimiters |
| bgroup(“(“,atop(x,y),”)”) | use scalable delimiters |
| group(lceil, x, rceil) | special delimiters |
| group(lfloor, x, rfloor) | special delimiters |
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