为什么用 pyplot 打印出来的图片日期是 3 个月一个间隔？而不是每个月

import csv
import matplotlib.pyplot as plt
from datetime import datetime

filename = 'death_valley_2014.csv'
with open(filename) as f:
    reader = csv.reader(f)
    header_row = next(reader)
    highs,dates,lows = [],[],[]
    for item in reader:
        try:
            time = datetime.strptime(item[0],'%Y-%m-%d')
            dates.append(time)
            high = item[1]
            low = item[3]
        except ValueError:
            print(time,"missing data")
        else:
            highs.append(high)
            lows.append(low)
fig = plt.figure(dpi=128,figsize = (18,10))
plt.plot(dates,highs,c='red')
plt.plot(dates,lows,c='blue')
plt.fill_between(dates,highs,lows,facecolor="blue",alpha=0.1)
plt.show()

dates 这个 list 中是 1 月到 12 月，但是 x 轴只显示 1、3、5、7、9、11 怎么让他每个都显示？