Python3使用matplotlib绘图时,坐标轴刻度不从X轴、y轴两端开始

import csv
from datetime import datetime
from matplotlib import pyplot as plt
#从文件中获取日期、气温
filename='death_valley_2014.csv'
with open(filename) as f:
    reader = csv.reader(f) 
    header_row = next(reader)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
    '''for index,column_header in enumerate(header_row):
        print(index,column_header)'''
    dates, highs, lows=[], [], []
    for row in reader:
        try:
            current_date=datetime.strptime(row[0],"%Y-%m-%d")
            high=int(row[1])
            low=int(row[3])
        except ValueError:
            print(current_date, 'missing data')
        else:
            dates.append(current_date)
            highs.append(high)
            lows.append(low)
# 根据数据绘制图形
fig = plt.figure(dpi=128, figsize= (10,6))
plt.plot(dates, highs, c='red')
plt.plot(dates, lows, c='blue')
plt.fill_between(dates, highs, lows, facecolor='blue', alpha=0.1)
# 设置图形的格式
plt.title("Daily high and low temperatures-2014", fontsize=24)
plt.xlabel('', fontsize=16)
fig.autofmt_xdate()
plt.ylabel('Temperatur(F)',fontsize = 16)
plt.tick_params(axis= 'both', which= 'major',labelsize= 16)
plt.show()                                                                                                                  

import matplotlib.pyplot as plt
import random
# 数据生成
x = range(50)
y = [random.uniform(15, 30) for i in x]
# 画布生成
plt.figure(figsize=(20, 8), dpi=80)
# 坐标轴添加
y_ticks = range(30)
plt.yticks(y_ticks[::5])   # y轴从0到30，间隔为5
# 图形绘制
plt.plot(x, y)
# 图形显示
plt.show()

import matplotlib.pyplot as plt
import random
# 数据生成
x = range(50)
y = [random.uniform(15, 30) for i in x]
# 画布生成
plt.figure(figsize=(20, 8), dpi=80)
# 图形绘制
plt.plot(x, y)
# 坐标轴添加
y_ticks = range(30)
plt.yticks(y_ticks[::5])
# 图形显示
plt.show()