7.3 Math Expression in Figures

Main idea: to use expression or convert latex to expression;

Two options:

expression(CO[2]) show math equation style as $CO_2$.

https://www.dataanalytics.org.uk/axis-labels-in-r-plots-using-expression/#expression_comm

Basic syntax:

^ for superscript

Superscript is “started” by the caret ^ character. Anything after ^ is superscript. The superscript continues until you use a * or ~ character.

If you want more text as superscript, then enclose it in quotes.
```
expression(Super^script~text)  # superscript ends at the space
```
renders as $Super^{script} text$.
```
expression(Super^"script text") # superscript continues until the closing quote
```
renders as $Super^{script text}$.

[] for subscript

plot(1,1, main=expression('title'^2))   # superscript
plot(1,1, main=expression('title'[2]))  # subscript

Use ~ for spacing in math mode.

When you type an expression() any spaces you type are ignored.
Use * for tight concatenation (no space).
Combine text + math: expression("Mean of " * mu) → “Mean of μ”.
- Quotes are optional. The usual letters and numbers are not “interpreted”.
- Quotes are used to enclose items that would otherwise be treated as a special character (like ~, *, +, -).
Font face: plain(), bold(), italic(), underline(), bolditalic().

Commonly used expressions:

λ	`expression(lambda)`
y $\le$ 5	`expression("y"<= "5")`
y$_i$	`expression(y[i])`
x$^2$	`expression(x^2)`
x$^2$ + 5	`expression(x^2~"+ 5"))` to add characters after a superscript, end the superscript with a `*` (no space) or `~` (space)
P(Y < y \| ɸ)	`expression(paste("P(Y", \<, "y \| ", phi,")"))` … expression(paste(“blah”, symbol, “blah”, symbol, “blah))

latex2exp::TeX("$\\alpha^\\beta$") show as $\alpha^{\beta}$;

Basic syntax:
- Escape special characters
  - Use \ before special characters, e.g., greek letters and latex commands that start with \;
  - Alternatively, use TeX(r"(\alpha)") to specify raw R string.
    
    r"( ... )" → everything inside is taken literally (no escaping). It means “take everything literally, don’t treat \ as escape characters.” ... can contain any character sequence, including \.
    
    See ?Quotes for raw string literal syntax.
```
library(latex2exp)

TeX(r"(\alpha)")       # raw string
TeX("\\alpha")         # same, but escaped
expression(alpha)      # equivalent
```
  All three produce the symbol α.
- Put math needs to be rendered between $...$ and regular text outside $...$ .
Tricks:
- Print whitespace in math mode $...$ :
  - use \\, or \\; to show white space in math mode. \\; is a larger space than \\,.
  - ~ to print a normal space
  - \phantom{x} to print a space the width of the character x.

# There is an R package called latex2exp which may be helpful. It has function TeX which accepts some LaTeX expressions enclosed with dollar sign $ as in this example:

library(latex2exp)
library(ggplot2)

qplot(1, "A")+
     ylab(TeX("Formula: $\\frac{2hc^2}{\\lambda^\\beta}$"))+
     xlab(TeX("$\\alpha$"))

TeX only put the part that needs latex interpretation between $...$ , and there are several escape characters that needs to be carefully treated.

Symbol	TeX
`\`	`\\`
`[`	`\[`
`]`	`\]`

Check out other escaping characters:

https://stat.ethz.ch/R-manual/R-devel/library/grDevices/html/plotmath.html

Ref:

latex2exp: a package to render LaTeX in R graphics

TeX in legend scales, the use of unname() is necessary:

# have to use unname(TeX("$...$"))
labels=c("CRU", unname( TeX("$Burke \\times 10$") ) )

When using TeX inside geom_text(aes(x, y, label=TeX("", output = "character") ), parse=TRUE ),

specifying the output of TeX() as character, although “character” is not one of the values that the output argument can take, and
turning the parse argument in geom_text() to TRUE.

Using variables in aes

aes_string(x="TCS_reported", y=as.name(target_v))

target_v can be a variable which will be parsed in aes;
as.name() first coerces its argument internally to a character vector; then takes the first element and returns a symbol of that name.
- A name (also known as a ‘symbol’) is a way to refer to R objects by name (rather than the value of the object, if any, bound to that name).
- It will escape special characters that are otherwise reserved or illegal; equivalent to 'target_v'

target_v <- "#Obs"
aes_string(x="TCS_reported", y=as.name(target_v))

# or equivalently replace `as.name()` with backticks ``
target_v <- "`#Obs`"
aes_string(x="TCS_reported", y=target_v)

# or using get()
aes(x=TCS_reported, y=get(target_v))

λ	`expression(lambda)`
y \(\le\) 5	`expression("y"<= "5")`
y\(_i\)	`expression(y[i])`
x\(^2\)	`expression(x^2)`
x\(^2\) + 5	`expression(x^2~"+ 5"))` to add characters after a superscript, end the superscript with a `*` (no space) or `~` (space)
P(Y < y \| ɸ)	`expression(paste("P(Y", \<, "y \| ", phi,")"))` … expression(paste(“blah”, symbol, “blah”, symbol, “blah))