bm.data = rbind(bm.data, cbind(x = x <- x + rnorm(n * grp), y = y <- y + rnorm(n * grp), step = i, group = rep(1:grp, each =                    n), id = rep(1:n, grp)))

print(qplot(x, y, facets = ~group, geom = "line", colour = id, alpha = I(0.1),
+                                data = subset(bm.data, step <= i), main = paste("step", i)) +
+               xlim(range(bm.data$x)) + ylim(range(bm.data$x)) +
+               geom_point(aes(x = x, y = y, facets = ~group, size = I(rep(3, n * grp))),
+                                data = subset(bm.data, step == i)) +
+               theme_bw() + theme(legend.position = "none"))

出现错误：
Don't know how to automatically pick scale for object of type formula. Defaulting to continuous
Don't know how to automatically pick scale for object of type AsIs. Defaulting to continuous
错误: Aesthetics must either be length one, or the same length as the dataProblems:~group
有哪位大神知道是什么错误吗？求教！