ggplot2制作第四幅图表

    ??theme_bw()
    
    ##用法，其他的几个内部参数相同
    theme_grey(
      base_size = 15,##整体布局字的缩放比例
      base_family = "bold",##所有的字体
      base_line_size = 1/10,##基本的线的粗细
      base_rect_size = 1/2
    )
    
    mtcars2 <- within(mtcars, {
      vs <- factor(vs, labels = c("V-shaped", "Straight"))
      am <- factor(am, labels = c("Automatic", "Manual"))
      cyl  <- factor(cyl)
      gear <- factor(gear)
    })
    
    p1 <- ggplot(mtcars2) +
      geom_point(aes(x = wt, y = mpg, colour = gear)) +
      labs(title = "Fuel economy declines as weight increases",