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I have a figure with a legend. I want the legend on the right with the legend title on the right of the legend. I also want the legend in the middle of the page.

Okay, first without a title. 

# Load library
library(ggplot2)

# Plot data without title
ggplot(iris) + 
  geom_point(aes(x = Sepal.Length, y = Sepal.Width, fill = Species), shape = 21) +
  theme(legend.title = element_text(angle = -90)) +
  guides(fill = guide_legend(title.position = "right",
                             title = ""))

Perfect! Now, just to add the title...

# Plot data
ggplot(iris) + 
  geom_point(aes(x = Sepal.Length, y = Sepal.Width, fill = Species), shape = 21) +
  theme(legend.title = element_text(angle = -90)) +
  guides(fill = guide_legend(title.position = "right",
                             title = "This is the species... Yup, defo the species"))

Hmmm. The legend is not centred relative to the title and page, so I use legend.title.align to try and rectify it.

# Attempted solutions
#1) legend.title.align
ggplot(iris) + 
  geom_point(aes(x = Sepal.Length, y = Sepal.Width, fill = Species), shape = 21) +
  theme(legend.title = element_text(angle = -90), legend.title.align = 0.5) + # 0.5 should be in the middle
  guides(fill = guide_legend(title.position = "right",
                             title = "This is the species... Yup, defo the species"))

No dice. Next try: title.vjust to vertically adjust the title.

#2) vjust
ggplot(iris) + 
  geom_point(aes(x = Sepal.Length, y = Sepal.Width, fill = Species), shape = 21) +
  theme(legend.title = element_text(angle = -90)) +
  guides(fill = guide_legend(title.position = "right",
                             title = "This is the species... Yup, defo the species",
                             title.vjust = 0.5)) # Vertical adjustment in figure

Not the solution either. Perhaps vjust and hjust are relative to the reading direction? So, let's try title.hjust...

#3) hjust - Perhaps it's relative to the direction of reading?
ggplot(iris) + 
  geom_point(aes(x = Sepal.Length, y = Sepal.Width, fill = Species), shape = 21) +
  theme(legend.title = element_text(angle = -90)) +
  guides(fill = guide_legend(title.position = "right",
                             title = "This is the species... Yup, defo the species",
                             title.hjust = 0.5)) # Horizontal adjustment... relative to reading direction?

That doesn't work either. Finally, perhaps vjust and hjust in element_text that defines legend_title are the correct parameters... 

#4) v/hjust in element_text
ggplot(iris) + 
  geom_point(aes(x = Sepal.Length, y = Sepal.Width, fill = Species), shape = 21) +
  theme(legend.title = element_text(angle = -90, hjust = 0.5, vjust = 0.5)) +
  guides(fill = guide_legend(title.position = "right",
                             title = "This is the species... Yup, defo the species"))

Created on 2019-10-21 by the reprex package (v0.3.0)

Nope. I'm sure this is straightforward and there are an abundance of somewhat similar solutions, yet I cannot find anything that works. Can someone explain my daft mistake?

Dan
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    Have you tried something like the solution presented here: https://stackoverflow.com/questions/48000292/center-align-legend-title-and-legend-keys-in-ggplot2-for-long-legend-titles – Brian Fisher Oct 21 '19 at 19:42
  • @BrianFisher Can you clarify which solution you mean – the accepted answer or the answer with the bounty – and also explain how it would work with rotation? – Dan Oct 21 '19 at 20:00
  • Not an expert in this regard, but had come across the referenced post. Both the accepted and bountied solutions had to dive into the source code and develop functions specifically to center the legend title and legend entries relative to each other. These solutions would probably need to be adapted to work in a rotated fashion, but seemed like a starting point. They also suggest your question probably doesn't have a straightforward solution. Hence a comment rather than an answer – Brian Fisher Oct 21 '19 at 20:27
  • @BrianFisher Okay, thanks for the thoughts – they're much appreciated. Unfortunately, I'm struggling to apply those solutions to the rotated case. I'm also a bit surprised that there isn't a simple fix given the number of parameters that are available that seemingly refer to the legend title. I could probably workaround the issue with `annotate` and switching clipping off, but I expect that solution would have to be rehashed for each use. – Dan Oct 21 '19 at 20:39

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