Positioning legend horizontally within a plot?

[Thomas]

I would like to put a legend horizontally in the plot area.  I am failing. If I set legend.position to a set of coordinates, it won't produce a horizontal legend.  If, I set legend position to "bottom", it will produce a horizontal legend, but now (of course) it is outside of the plot area).  Any help with this mystery would be appreciated. (note, the real data has negative values so scale_colour_gradient2 will display the red)  

p <- ggplot(diamonds, aes(x, y))+

  geom_point(aes(colour=carat))+

  scale_colour_gradient2(low="red",mid="white",high="blue", name="this is\nthe legend")+  

  theme(legend.text = element_text(size=12),

        legend.key.size = unit(1.5, "lines"),

        legend.key = element_blank(),

        legend.position = c(0.1,0.17), 

        #legend.position = "bottom",

        legend.box = "horizontal"  ) 

x11()

p

[Ista Zahn]

Hi Thomas,

The element should be legend.direction, not legend.box.

Best,

Ista

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[Thomas]

That worked, thanks.  I now have another problem with this.  The legend title will only appear to the left of the horizontal legend.  I have seen a number of examples with horizontal legends and titles on top, but they never have explicit code for doing this? 

In the following code I want the legend title on top of the color scale.

p <- ggplot(diamonds, aes(x, y))+

  geom_point(aes(colour=carat))+

  scale_colour_gradient2(low="red",mid="white",high="blue", name="this is the legend")+  

  theme(

        legend.text = element_text(size=12),

        legend.key = element_blank(),

        legend.position = c(0.1,0.17), 

        legend.direction = "horizontal"  ) 

x11()

p

[baptiste auguie]

p + guides(colour=guide_colourbar(title.position="top"))

HTH,

b.

[Thomas]

Thanks, 

That seems to resolve that issue. Last question is:  How do I center the entire legend?

The below code center the title, but if the title is greater than the length of the legend, it is a left justified center.  I can fake a solution by just making a longer bar (see commented out code), 

but I am curious to know the real solution. I didn't see anything like "colourbar.hjust"...

p <- ggplot(diamonds, aes(x, y))+

  geom_point(aes(colour=carat))+

  scale_colour_gradient2(low="red",mid="white",high="blue", name="this is the longer version for \nlegend")+  

  guides(colour=guide_colourbar(title.position="top", title.hjust=0.5))+ #centered title but the legend is left justified overall

  #guides(colour=guide_colourbar(title.position="top", title.hjust=0.5, barwidth=10))+ #fake solution to making entire legend appear centered.

  theme(

        legend.text = element_text(size=12),

        legend.key = element_blank(),

        legend.position = c(0.4,0.17),