# 【2.8.3】图形填充fill/fill_between

## 一、绘制填充多边形fill()

### 1.1 语法结构

fill(*args, **kwargs)

args - sequence of x, y, [color]

ax.fill(x, y)                    # a polygon with default color
ax.fill(x, y, "b")               # a blue polygon
ax.fill(x, y, x2, y2)            # two polygons
ax.fill(x, y, "b", x2, y2, "r")  # a blue and a red polygon


kwargs - 对象matplotlib.patches.Polygon的特性（class:~matplotlib.patches.Polygon properties）

## 1.2 示例

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 5 * np.pi, 1000)

y1 = np.sin(x)
y2 = np.sin(2 * x)

plt.plot(x, y1, label="$y = sin(x)$")
plt.plot(x, y2, label="$y = sin(2 * x)$")
plt.legend(loc=3)

plt.show()


import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 5 * np.pi, 1000)

y1 = np.sin(x)
y2 = np.sin(2 * x)

plt.fill(x, y1, color="g", alpha=0.3)
plt.fill(x, y2, color="b", alpha=0.3)

plt.show()


## 二、函数间区域填充fill_between

### 2.1 基本语法

plt.fill_between(
x, y1, y2=0, where=None,
interpolate=False, step=None,
hold=None, data=None,
**kwargs
)

x - array( length N) 定义曲线的 x 坐标

y1 - array( length N ) or scalar 定义第一条曲线的 y 坐标

y2 - array( length N )  or scalar 定义第二条曲线的 y 坐标

where - array of bool (length N), optional, default: None

排除一些（垂直）区域被填充。

注：我理解的垂直区域，但帮助文档上写的是horizontal regions


plt.fill_between(x，y1，y2，where=条件表达式, color=颜色，alpha=透明度)

" where = " 可以省略，直接写条件表达式


### 2.2 具体示例

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 5 * np.pi, 1000)

y1 = np.sin(x)
y2 = np.sin(2 * x)

plt.plot(x, y1, c="g")
plt.plot(x, y2, c='r')

# 将两函数间区域填充成浅灰色
plt.fill_between(x, y1, y2, facecolor="lightgray")

plt.show()


import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 5 * np.pi, 1000)

y1 = np.sin(x)
y2 = np.sin(2 * x)

plt.plot(x, y1, c="y")
plt.plot(x, y2, c='r')

# 将两函数间区域填充成浅灰色
plt.fill_between(x, y1, y2, facecolor="lightgray")

plt.show()


import numpy as np
import matplotlib.pyplot as plt
n = 1000
x = np.linspace(0, 8 * np.pi, n)
sin_y = np.sin(x)
cos_y = np.cos(x / 2) / 2

plt.figure('Fill', facecolor='lightgray')
plt.title('Fill', fontsize=20)
plt.xlabel('x', fontsize=14)
plt.ylabel('y', fontsize=14)
plt.tick_params(labelsize=10)
plt.grid(linestyle=':')

# 把正弦余弦两条曲线画出
plt.plot(x, sin_y, c='dodgerblue', label=r'$y=sin(x)$')
plt.plot(x, cos_y, c='orangered',
label=r'$\frac{1}{2}cos(\frac{x}{2})$')

# 填充
plt.fill_between(x, cos_y, sin_y, cos_y < sin_y,
color='dodgerblue', alpha=0.5)
plt.fill_between(x, cos_y, sin_y, cos_y > sin_y,
color='orangered', alpha=0.5)

plt.legend(loc = 3)
plt.show()