How?

To use JuliaCall package for julia engine in R Markdown document. Just set the engine for julia to JuliaCall::eng_juliacall like this:

knitr::opts_chunk$set(echo = TRUE)
knitr::knit_engines$set(julia = JuliaCall::eng_juliacall)

And in the julia code chunk, just use “{julia}" or "{r, engine=‘julia’}”.

Below is how it looks like when using JuliaCall as language engine for Julia.

How Does JuliaCall in R Markdown Look Like?

## This is a julia language chunk.
## In julia, the command without ending semicolon will trigger the display
## so is JuliaCall package. 
## The julia display will follow immediately after the corresponding command
## just as the R code in R Markdown.
a = sqrt(2)
1.4142135623730951 
a = sqrt(2);

## And lots of different types of display are supported in JuliaCall.
## Like markdown and plots.
@doc sqrt
sqrt(x)

Return ${x}$. Throws DomainError for negative Real arguments. Use complex negative arguments instead. The prefix operator √ is equivalent to sqrt. 

using Gadfly
Gadfly.plot(y = [1 2 3])
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using Plots
pyplot()
Plots.PyPlotBackend() 
Plots.plot(sin,(x->begin sin(2x) end),0,2pi,line=4,leg=false,fill=(0,:orange))

Get Access to Julia in R Chunk

And you can also get access to julia variables in R code chunk quite easily using JuliaCall, for example:

## This is a R language chunk.
## In the previous julia chunk, we define variable a, 
## we can use functions in JuliaCall to get access to it.
JuliaCall::julia_eval_string("a")
## [1] 1.414214
## And you can also get access to Julia libraries and functions.
JuliaCall::julia_library("Gadfly")
JuliaCall::julia_call("Gadfly.plot", y = c(1:3), 
                      need_return = FALSE, show_value = TRUE)
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JuliaCall::julia_library("Plots")
JuliaCall::julia_command("pyplot()")
Plots.PyPlotBackend() 
JuliaCall::julia_command("Plots.plot(sin,(x->begin sin(2x) end),0,2pi,line=4,leg=false,fill=(0,:orange))")

Current Limitations

Currently only R Markdown html output is fully supported by JuliaCall.

Custom Style of Julia Display in R Markdown Using CSS

When using JuliaCall in R Markdown, every julia display will be wrapped in a div with class = "JuliaDisplay", so you could customize the looking of julia display using css, for example, I use

    <style>
    .JuliaDisplay {
        color: blue;
    }
    </style>

in this document.