HTML greek letters with ggplotly

Question

When converting a ggplot graphics to a plotly graphics with ggplotly, I can get some HTML, for example in the title:

library(plotly)
library(ggplot2)
library(scales)

library(mvtnorm)
sims <- rmvnorm(500, mean=c(0,0,0))
dat <- setNames(data.frame(sims), c("x", "y", "z"))
dat$z <- dat$x+dat$y

gg <- ggplot(dat, aes(x=x, y=y, colour=z)) + 
  geom_point() + 
  scale_colour_gradient2(limits=c(-2,2), midpoint=0, low=muted("blue"), high=muted("red")) + 
  ggtitle("<em>hello</em>")

ggplotly(gg)

The title <em>hello</em> appears in italic, as expected.

But when I want a HTML greek letter, that does not work:

gg <- ggplot(dat, aes(x=x, y=y, colour=z)) + 
  geom_point() + 
  scale_colour_gradient2(limits=c(-2,2), midpoint=0, low=muted("blue"), high=muted("red")) + 
  ggtitle("&Delta;")

ggplotly(gg)

How to render Δ as the expected greek letter ?

I'm using plotly_4.5.2 and ggplot2_2.1.0.

Answer 1

This works with the decimal character reference:

gg <- ggplot(dat, aes(x=x, y=y, colour=z)) + 
  geom_point() + 
  scale_colour_gradient2(limits=c(-2,2), midpoint=0, low=muted("blue"), high=muted("red")) + 
  ggtitle("&#916;")
ggplotly(gg)

According to this comment on the Github repo, plotly.js recently removed generalized HTML entity decoding". I'm not sure to understand, but it could be the explanation.

Update 2017-08-25

The plotly developers have now removed the html decimal character encoding. As a solution, one can directly enter an UTF-8 character, or use intToUtf8:

ggtitle("Δ")
ggtitle(intToUtf8(0x0394L))