核密度图（直方图的拟合曲线）

核密度图可以看作是概率密度图，其纵轴可以粗略看做是数据出现的次数，与横轴围成的面积是一.

法一：seaborn的kdeplot函数专门用于画核密度估计图.

参考：https://www.jianshu.com/p/844f66d00ac1

https://yq.aliyun.com/articles/682843

import pandas as pd
import numpy as np
import matplotlib.pylab as plt
import os
import seaborn as sns

## 有时候要FQ才能下载
# df = pd.read_csv("https://github.com/selva86/datasets/raw/master/mpg_ggplot2.csv")
# df.to_csv('../data/mpg_ggplot2.csv', index=False)
df = pd.read_csv('../data/mpg_ggplot2.csv')

print(df.info())
print(df.shape)
# Draw Plot
plt.figure(figsize=(16,10), dpi= 90)  # dpi用于设置输出figure中所有字体的大小
# 将cyl列等于4的cty筛选出来做图
sns.kdeplot(df.loc[df['cyl'] == 4, "cty"], shade=True, color="g", label="Cyl=4", alpha=0.5)
sns.kdeplot(df.loc[df['cyl'] == 5, "cty"], shade=True, color="deeppink", label="Cyl=5", alpha=.5)
sns.kdeplot(df.loc[df['cyl'] == 6, "cty"], shade=True, color="dodgerblue", label="Cyl=6", alpha=.5)
sns.kdeplot(df.loc[df['cyl'] == 8, "cty"], shade=True, color="orange", label="Cyl=8", alpha=.5)
# Decoration
plt.title('Density Plot of City Mileage by n_Cylinders', fontsize=22)
plt.legend()
plt.show()

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