```r
p4 <- p3 + theme(
  ### remove axis ticks
  axis.ticks=element_blank(),
  ### modify axis lines
  axis.line.y = element_line(colour = "pink", size = 1, linetype = "dashed"),
  axis.line.x = element_line(colour = "pink", size = 1.2, linetype = "dashed"))
```