matplotlib：如何防止 x 轴标签重叠

Question

我正在使用 matplotlib 生成条形图。一切正常，但我不知道如何防止 x 轴的标签相互重叠。这里举个例子：

这里是一个 postgres 9.1 数据库的一些示例 SQL：

drop table if exists mytable;
create table mytable(id bigint, version smallint, date_from timestamp without time zone);
insert into mytable(id, version, date_from) values

('4084036', '1', '2006-12-22 22:46:35'),
('4084938', '1', '2006-12-23 16:19:13'),
('4084938', '2', '2006-12-23 16:20:23'),
('4084939', '1', '2006-12-23 16:29:14'),
('4084954', '1', '2006-12-23 16:28:28'),
('4250653', '1', '2007-02-12 21:58:53'),
('4250657', '1', '2007-03-12 21:58:53')
;  

这是我的 python 脚本：

# -*- coding: utf-8 -*-
#!/usr/bin/python2.7
import psycopg2
import matplotlib.pyplot as plt
fig = plt.figure()

# for savefig()
import pylab

###
### Connect to database with psycopg2
###

try:
  conn_string="dbname='x' user='y' host='z' password='pw'"
  print "Connecting to database\n->%s" % (conn_string)

  conn = psycopg2.connect(conn_string)
  print "Connection to database was established succesfully"
except:
  print "Connection to database failed"

###
### Execute SQL query
###  

# New cursor method for sql
cur = conn.cursor()

# Execute SQL query. For more than one row use three '"'
try:
  cur.execute(""" 

-- In which year/month have these points been created?
-- Need 'yyyymm' because I only need Months with years (values are summeed up). Without, query returns every day the db has an entry.

SELECT to_char(s.day,'yyyymm') AS month
      ,count(t.id)::int AS count
FROM  (
   SELECT generate_series(min(date_from)::date
                         ,max(date_from)::date
                         ,interval '1 day'
          )::date AS day
   FROM   mytable t
   ) s
LEFT   JOIN mytable t ON t.date_from::date = s.day
GROUP  BY month
ORDER  BY month;

  """)

# Return the results of the query. Fetchall() =  all rows, fetchone() = first row
  records = cur.fetchall()
  cur.close()

except:
  print "Query could not be executed"

# Unzip the data from the db-query. Order is the same as db-query output
year, count = zip(*records)

###
### Plot (Barchart)
###

# Count the length of the range of the count-values, y-axis-values, position of axis-labels, legend-label
plt.bar(range(len(count)), count, align='center', label='Amount of created/edited points')

# Add database-values to the plot with an offset of 10px/10px
ax = fig.add_subplot(111)
for i,j in zip(year,count):
    ax.annotate(str(j), xy=(i,j), xytext=(10,10), textcoords='offset points')

# Rotate x-labels on the x-axis
fig.autofmt_xdate()

# Label-values for x and y axis
plt.xticks(range(len(count)), (year))

# Label x and y axis
plt.xlabel('Year')
plt.ylabel('Amount of created/edited points')

# Locate legend on the plot (http://matplotlib.org/users/legend_guide.html#legend-location)
plt.legend(loc=1)

# Plot-title
plt.title("Amount of created/edited points over time")

# show plot
pylab.show()

有没有办法防止标签相互重叠？理想情况下以自动方式进行，因为我无法预测柱的数量。

Answer 1

• OP 中的问题是日期格式为string 类型。 matplotlib 将每个值绘制为刻度标签，刻度位置是基于值数量的 0 索引数字。
• 解决此问题的方法是将所有值转换为正确的type，在这种情况下datetime。
  • 一旦axes 具有正确的type，就会有额外的matplotlib methods，可用于进一步自定义刻度间距。
• What is plotted when string data is passed to the matplotlib API? 的答案更详细地解释了将 string 值传递给 matplotlib 时会发生什么。
• 截至2014-09-30，pandas有一个read_sql函数，它有一个parse_dates参数。您肯定想改用它。

原答案

以下是将日期字符串转换为真实日期时间对象的方法：

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.dates as mdates
data_tuples = [
    ('4084036', '1', '2006-12-22 22:46:35'),
    ('4084938', '1', '2006-12-23 16:19:13'),
    ('4084938', '2', '2006-12-23 16:20:23'),
    ('4084939', '1', '2006-12-23 16:29:14'),
    ('4084954', '1', '2006-12-23 16:28:28'),
    ('4250653', '1', '2007-02-12 21:58:53'),
    ('4250657', '1', '2007-03-12 21:58:53')]
datatypes = [('col1', 'i4'), ('col2', 'i4'), ('date', 'S20')]
data = np.array(data_tuples, dtype=datatypes)
col1 = data['col1']

# convert the dates to a datetime type
dates = mdates.num2date(mdates.datestr2num(data['date']))
fig, ax1 = plt.subplots()
ax1.bar(dates, col1)
fig.autofmt_xdate()

从数据库游标中获取一个简单的元组列表应该很简单......

data_tuples = []
for row in cursor:
    data_tuples.append(row)

但是，我在这里发布了一个函数版本，用于直接使用 db 游标来记录数组或 pandas 数据帧：How to convert SQL Query result to PANDAS Data Structure?

希望这也有帮助。

Answer 2

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
# create a random dataframe with datetimeindex
date_range = pd.date_range('1/1/2011', '4/10/2011', freq='D')
df = pd.DataFrame(np.random.randint(0,10,size=(100, 1)), columns=['value'], index=date_range)

日期刻度标签经常重叠：

plt.plot(df.index,df['value'])
plt.show()

因此旋转它们并右对齐它们很有用。

fig, ax = plt.subplots()
ax.plot(df.index,df['value'])
ax.xaxis_date()     # interpret the x-axis values as dates
fig.autofmt_xdate() # make space for and rotate the x-axis tick labels
plt.show()

Answer 3

关于如何在 xaxis 上仅显示每 4 个刻度（例如）的问题，您可以这样做：

import matplotlib.ticker as mticker

myLocator = mticker.MultipleLocator(4)
ax.xaxis.set_major_locator(myLocator)

Answer 4

我认为您对 matplotlib 如何处理日期的几点感到困惑。

目前，您实际上并没有计划日期。您正在使用 [0,1,2,...] 在 x 轴上绘制内容，然后使用日期的字符串表示手动标记每个点。

Matplotlib 会自动定位刻度。但是，您覆盖了 matplotlib 的刻度定位功能（使用 xticks 基本上是在说：“我想要这些位置的刻度”。）

目前，如果 matplotlib 自动定位它们，您将在 [10, 20, 30, ...] 处获得刻度。但是，这些将对应于您用于绘制它们的值，而不是日期（绘制时没有使用）。

您可能希望实际使用日期来绘制事物。

目前，您正在做这样的事情：

import datetime as dt
import matplotlib.dates as mdates
import numpy as np
import matplotlib.pyplot as plt

# Generate a series of dates (these are in matplotlib's internal date format)
dates = mdates.drange(dt.datetime(2010, 01, 01), dt.datetime(2012,11,01), 
                      dt.timedelta(weeks=3))

# Create some data for the y-axis
counts = np.sin(np.linspace(0, np.pi, dates.size))

# Set up the axes and figure
fig, ax = plt.subplots()

# Make a bar plot, ignoring the date values
ax.bar(np.arange(counts.size), counts, align='center', width=1.0)

# Force matplotlib to place a tick at every bar and label them with the date
datelabels = mdates.num2date(dates) # Go back to a sequence of datetimes...
ax.set(xticks=np.arange(dates.size), xticklabels=datelabels) #Same as plt.xticks

# Make space for and rotate the x-axis tick labels
fig.autofmt_xdate()

plt.show()

请尝试以下方法：

import datetime as dt
import matplotlib.dates as mdates
import numpy as np
import matplotlib.pyplot as plt

# Generate a series of dates (these are in matplotlib's internal date format)
dates = mdates.drange(dt.datetime(2010, 01, 01), dt.datetime(2012,11,01), 
                      dt.timedelta(weeks=3))

# Create some data for the y-axis
counts = np.sin(np.linspace(0, np.pi, dates.size))

# Set up the axes and figure
fig, ax = plt.subplots()

# By default, the bars will have a width of 0.8 (days, in this case) We want
# them quite a bit wider, so we'll make them them the minimum spacing between
# the dates. (To use the exact code below, you'll need to convert your sequence
# of datetimes into matplotlib's float-based date format.  
# Use "dates = mdates.date2num(dates)" to convert them.)
width = np.diff(dates).min()

# Make a bar plot. Note that I'm using "dates" directly instead of plotting
# "counts" against x-values of [0,1,2...]
ax.bar(dates, counts, align='center', width=width)

# Tell matplotlib to interpret the x-axis values as dates
ax.xaxis_date()

# Make space for and rotate the x-axis tick labels
fig.autofmt_xdate()

plt.show()