ggplot2 heatmap: side margins and color legend

Question

The following code is used to produce a correlation matrix heatmap in ggplot2

  set.seed(1)
  n  <- 10
  p  <- 100
  X  <- matrix(rnorm(n*p), ncol=p)
  R  <- cor(X)

  library("reshape2")
  x    <- melt(R)
  colnames(x) <- c('row', 'col', 'cor') 

  library("ggplot2")
  HM <- ggplot(x, aes(x=row, y=col, fill = cor)) + geom_tile()
  HM <- HM + scale_fill_gradient2(low='red', mid ='white', high ='blue', midpoint=0, guide = "colourbar")
  HM <- HM  + scale_x_discrete(expand = c(0, 0))
  HM <- HM  + scale_y_discrete(expand = c(0, 0))
  HM <- HM  + coord_equal()
  HM <- HM  + labs(title = "Correlation Matrix") [![enter image description here][1]][1]
  HM <- HM  + theme(plot.title = element_text(hjust = 0.5), 
    axis.text=element_blank(), 
    axis.ticks=element_blank(),
    axis.title=element_blank(),
    plot.margin = unit(c(0,0,0,0), "cm"))
  print(HM)

The code produce the following picture

It is nice, but I have two questions

1. when I print the pdf the white side margins are a waste of space especially when I put figure in a row. I tried to use mar and oma paramter in par() but this do not produce any difference.

2. the legend on the right does not cover the range [-1,1] this is because the min correlation in the matrix is larger than -1. But still I'd like to have the legend to code the whole range [-1,1]

3. if possible I'd like to have the legend box justified on top and not at the center of the vertical axes, I mean I'd like it starts from north-east corner down.

Any ideas?

Cheers Pierre