7.3 Math Expression in Figures

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

Two options

  1. expression(CO[2]) show math equation style as \(CO_2\). https://www.dataanalytics.org.uk/axis-labels-in-r-plots-using-expression/#expression_comm

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

Trick: use \\, or \\; to show white space in math mode. \\; is a larger space than \\,.

# 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

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

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))