在R中按分位数绘制

Question

I want to plot foo ~ bar . However, I don't want to look at the exact data, I'd rather break bar into say quantiles, and plot mean(foo) for every quantile (so my final plot will have 5 data points). Is this possible?

Answer 1

 foo <- rnorm(100)
 bar <- rnorm(100)

  mn.foo.byQ10bar <- tapply(foo, cut(bar, quantile(bar, (0:5)/5, na.rm=TRUE)), mean)

> mn.foo.byQ5bar
 (-3.31,-0.972] (-0.972,-0.343]  (-0.343,0.317]   (0.317,0.792]    (0.792,2.71] 
     0.13977839      0.03281258     -0.18243804     -0.14242885     -0.01696712 

 plot(mn.foo.byQ5bar)

This is a fairly standard task and Harrell's Hmisc package's cut2 function has a nice gr= argument that lets you do this by just specifying an integer for the number of groups. I also like it because the intervals from the cut operation are left-closed instead of R default for right-closed.

Answer 2

You can combine a lot of these lines into more concise code, but here it is broken down

# Sample Data: 
x <- 1:100;   y <- rnorm(x)

# Number Of Groups
N <- 5

# quantiles
Q.y <- quantile(y, probs=seq(0, 1, length=(N+1)))
Q.x <- quantile(x, probs=seq(0, 1, length=N))

# means of y by quantile
means.y <- c(by(y, cut(y, Q.y), mean))

# plot them 
qplot(Q.x, means.y)