Re-add alpha scale

R/scale-continuous-alpha.r

@@ -0,0 +1,32 @@
+ScaleAlpha <- proto(ScaleContinuous, expr={
+  doc <- TRUE
+  common <- NULL
+
+  new <- function(., name=NULL, limits=NULL, breaks=NULL, labels=NULL, trans = NULL, to = c(0.1, 1)) {
+    .super$new(., name=name, limits=limits, breaks=breaks, labels=labels, trans=trans, variable = "alpha", to = to)
+  }
+
+  map <- function(., values) {
+    rescale(values, .$to, .$input_set())
+  }
+  output_breaks <- function(.) .$map(.$input_breaks())
+
+  objname <- "alpha"
+  desc <- "Alpha scale for continuous variable"
+
+  icon <- function(.) {
+    x <- c(0.1, 0.3, 0.5, 0.7, 0.9)
+    rectGrob(x, width=0.25, 
+      gp=gpar(fill=alpha("black", x), col=NA)
+    )
+
+  }
+
+  examples <- function(.) {
+    (p <- qplot(mpg, cyl, data=mtcars, alpha=cyl))
+    p + scale_alpha("cylinders")
+    p + scale_alpha("number\nof\ncylinders")
+
+    p + scale_alpha(to = c(0.4, 0.8))
+  }  
+})