请问一下为什么x轴的数字排序混乱了

Question

import json
import pygal
import math
from itertools import groupby
#将数据加载到一个列表中
filename = 'btc_close_2017.json'
with open(filename) as f:
    btc_date = json.load(f)
#创建5个列表，分别存储日期和收盘价
dates,months,weeks,weekdays,close = [],[],[],[],[]
#打印每一天的信息
for btc_dict in btc_date:
    dates.append(btc_dict['date'])
    months.append(btc_dict['month'])
    weeks.append(btc_dict['week'])
    weekdays.append(btc_dict['weekday'])
    close.append(float(btc_dict['close']))
#绘制收盘价折线图
line_chart = pygal.Line(x_label_rotation=20, show_minor_x_labels=False)
line_chart.title = '收盘价对数变换'
line_chart.x_labels = dates
N = 20 #x轴坐标每隔20 显示
line_chart.x_labels_major = dates[::N]
close_log = [math.log10(_) for  _ in close]
line_chart.add('log收盘价',close_log)
line_chart.render_to_file('收盘价对数变换折线图.svg')
def draw_line(x_data, y_data, title, y_legend):
    xy_map = []
    for x, y in groupby(sorted(zip(x_data, y_data)), key=lambda _: _[0]):
        y_list = [v for _, v in y]
        xy_map.append([x, sum(y_list) / len(y_list)])
    x_unique, y_mean = [*zip(*xy_map)]
    line_chart = pygal.Line()
    line_chart.title = title
    line_chart.x_labels = x_unique
    line_chart.add(y_legend, y_mean)
    line_chart.render_to_file(title+ '.svg')
    return line_chart
idx_month = dates.index('2017-12-01')
line_chart_month = draw_line(months[:idx_month], close[:idx_month], '收盘价月日均值', '月日均值')
line_chart_month
idx_week = dates.index('2017-12-11')
line_chart_week = draw_line(weeks[1:idx_week], close[1:idx_week], '收盘价周日均值', '周日均值')
line_chart_week

Answer 1

1 ==> 01
2 ==> 02
.
.
..