因为你的x轴不是continuous的变量，y轴是。
qplot(color,data=diamonds,geom="bar",weight=carat) + scale_x_discrete("Color")

如果楼主想改坐标轴的标题的话，可以这样写：
qplot(color,data=diamonds,geom="bar",weight=carat) +labs(y="克拉",x="颜色")

好像是起到修饰图形的作用，详细的用法是
scale_y_continuous(name=NULL, limits=NULL, breaks=NULL, labels=NULL, trans=NULL, expand=c(0.05, 0), minor_breaks=NULL, formatter="scientific", ...)
name: name of scale to appear in legend or on axis. Maybe be an expression: see ?plotmath
limits: numeric vector of length 2, giving the extent of the scale
breaks: numeric vector indicating where breaks should lie
labels: character vector giving labels associated with breaks
trans: a transformer to use
expand: numeric vector of length 2, giving multiplicative and additive expansion factors
minor_breaks: NULL
formatter: NULL
...: ignored


比如
(m <- qplot(rating, votes, data=subset(movies, votes > 1000), na.rm = T))
m + scale_y_continuous(limits=c(0, 5000))

老兄，这样好像不行，我做的结果是简单的条图，也就是说，纵轴仍是count,而不是目标的carat

那你就用
qplot(color, data = diamonds, geom = "bar")+scale_y_continuous("count")
或者
qplot(color, data = diamonds, geom = "bar")
即可！