mtcars
R的内置数据集mtcars 记录了32种不同品牌的轿车的的11个属性，如下：
[, 1]
mpg
Miles/(US) gallon
[, 2]
cyl
Number of cylinders
[, 3]
disp
Displacement (cu.in.)
[, 4]
hp
Gross horsepower
[, 5]
drat
Rear axle ratio
[, 6]
wt
Weight (lb/1000)
[, 7]
qsec
1/4 mile time
[, 8]
vs
V/S
[, 9]
am
Transmission (0 = automatic, 1 = manual)
[,10]
gear
Number of forward gears
[,11]
carb
Number of carburetors