R qqman manhattan png

Question

我从CRAN Berkeley http存储库运行了R包'qqman'。我使用fread来读取GWAS摘要统计信息。输出是一个33.5MB的PDF文件，慢慢“绘制”绘图并崩溃Adobe。为什么输出PDF而不是简单的PNG？

options(stringsAsFactors = FALSE)  
library(data.table)   
library(qqman)  

c <- fread("path", header = TRUE, data.table=FALSE)  

manhattan(c, chr = "CHR", bp = "BP", p = "P", snp = "SNP", col = c("blue", "cyan"), 
  chrlabs = NULL, suggestiveline = -log10(1e-05), genomewideline = -log10(5e-08), 
  highlight = NULL, logp = TRUE, annotatePval = NULL, annotateTop = TRUE)