用pygal画折线图日期横坐标显示...无法正常显示日期，代码如下
import json
import pygal
filename = 'btc_close_2017_urllib.json'
with open(filename) as f:
btc_data = json.load(f)
dates = []
months = []
weeks = []
weekdays = []
close = []
for btc_dict in btc_data:
#current_date = datetime.strptime(btc_dict['date'],'%Y-%m-%d')
dates.append(btc_dict['date'])
months.append(int(btc_dict['month']))
weeks.append(int(btc_dict['week']))
weekdays.append(btc_dict['weekday'])
close.append(int(float(btc_dict['close'])))
line_chart = pygal.Line(show_minor_x_labels=False)
line_chart.title = '收盘价(￥)'
line_chart.x_labels = dates
line_chart.x_labels_major = dates[::20]
line_chart.add('收盘价',close)
line_chart.render_to_file('收盘价折线图(￥).svg')
结果见图，是啥原因呢？求大佬解答。
图片说明
