R语言操作(UDA)-- Part5: 钻石与价格预测

5.1 创建一个散点图，查看carat和price之间是否存在线性关系

Let's start by examining two variables in the data set.
The scatterplot is a powerful tool to help you understand
the relationship between two continuous variables.

We can quickly see if the relationship is linear or not.
In this case, we can use a variety of diamond
characteristics to help us figure out whether
the price advertised for any given diamond is
reasonable or a rip-off.

Let's consider the price of a diamond and it's carat weight.
Create a scatterplot of price (y) vs carat weight (x).

Limit the x-axis and y-axis to omit the top 1% of values.

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ggplot(aes(x = carat, y = price), data = diamonds) +
geom_point() +
xlim(0, quantile(diamonds$carat, 0.99)) +
ylim(0, quantile(diamonds$price, 0.99))

可见，carat和price之间呈现非线性关系，类似于指数函数；并且随着carat的尺寸增加，散点更加离散。

我们加上一条线性趋势线再来看：
ggplot(aes(x = carat, y = price), data = diamonds) +
geom_point() +
xlim(0, quantile(diamonds$carat, 0.99)) +
ylim(0, quantile(diamonds$price, 0.99)) +
geom_point(fill = I('#F79420'), color = I('black'), shape = 21) +
stat_smooth(method = 'lm')