ggplot2 specific command

# By default, the same scales are used for all panels. You can allow
# scales to vary across the panels with the `scales` argument.
# Free scales make it easier to see patterns within each panel, but
# harder to compare across panels.
#通过实践该代码即可领悟
##1
ggplot(mpg, aes(displ, hwy)) + geom_point() +facet_wrap(~class, scales = "free")
ggplot(mpg, aes(displ, hwy)) + geom_point() +facet_wrap(~class)

# To repeat the same data in every panel, 
# simply construct a data frame that does not contain the facetting variable.
#通过实践该代码即可领悟
ggplot(mpg, aes(displ, hwy)) + geom_point(data = transform(mpg, class = NULL), colour = "grey85") + geom_point() + facet_wrap(~class)

ggplot(mpg, aes(displ, hwy)) + geom_point() + facet_wrap(~class)

#labs() ----Modify axis, legend, and plot labels
#ggplot2 绘图时使用该参数来调整坐标轴、图例和标签

#reorder 根据数值型变量对分类变量进行重新排序
#reorder is a generic function. The "default" method treats its first argument as a categorical variable, and reorders its levels based on the values of a second variable, usually numeric.