ggplot轴刻度标签被切断

Question

我正在制作geom_tile ggplot，我的x轴刻度标签很长：

set.seed(1)
df <- data.frame(sample = c(rep(paste(rep("S1",30),collapse=""),30),
                            rep(paste(rep("S2",30),collapse=""),30),
                            rep(paste(rep("S3",30),collapse=""),30)),
                 gene = rep(paste("G", 1:30, sep = ""), 90),
                 value = rnorm(90))

因此，当我制作情节时，他们会被切断：

require(ggplot2)

ggplot(df, aes(x = sample, y = gene)) + 
    geom_tile(aes(fill = value)) + 
    scale_fill_gradient2(high = "darkred", low = "darkblue") +
    theme_bw() + 
    theme(legend.key = element_blank(), 
          legend.position = "right", 
          axis.text.y = element_blank(), 
          axis.ticks.y = element_blank(), 
          panel.border = element_blank(), 
          strip.background = element_blank(), 
          axis.text.x = element_text(angle = 45, hjust = 1, vjust = 1))

我该如何解决这个问题？