plot(x,y,"l")
Arguments to be passed to methods, such as graphical parameters (see par). Many methods will accept the following arguments:

type
what type of plot should be drawn. Possible types are

"p" for points,

"l" for lines,

"b" for both,

"c" for the lines part alone of "b",

"o" for both ‘overplotted’,

"h" for ‘histogram’ like (or ‘high-density’) vertical lines,

"s" for stair steps,

"S" for other steps, see ‘Details’ below,

"n" for no plotting.

参数“l”是把所有点连起来，没办法画出二次函数的拟合曲线

fit <- lm(y~x+I(x^2))
plot(x,y)
lines(x,fitted(fit))

画不出拟合的二次曲线