用python Linux（无GUI）中生成统计图

Question

我有这样一个需求：在我的raspberry pi 3中运行着一个爬虫程序，根据爬到的数据，使用matplotlib自动生成统计图（图片格式），然后发送到我的邮箱.

但是我ssh远程登录树莓派，运行如下脚本生成test.png图片时

import matplotlib.pyplot as plt
import numpy as np
import matplotlib

t = np.arange(0.0, 10.0, 0.001)
s = np.sin(2*np.pi*t)
plt.plot(t, s)

plt.xlabel('time (s)')
plt.ylabel('voltage (mV)')
plt.title('About as simple as it gets, folks')
plt.grid(True)
plt.savefig("test.png")

出现如下错误：

(process:2818): Gtk-WARNING **: Locale not supported by C library.
           Using the fallback 'C' locale.
Unable to init server: Could not connect: Connection refused
Unable to init server: Could not connect: Connection refused

(simple_plot.py:2818): Gdk-CRITICAL **: gdk_cursor_new_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
Traceback (most recent call last):
  File "simple_plot.py", line 1, in <module>
    import matplotlib.pyplot as plt
  File "/usr/local/lib/python2.7/dist-packages/matplotlib/pyplot.py", line 114, in <module>
    _backend_mod, new_figure_manager, draw_if_interactive, _show = pylab_setup()
  File "/usr/local/lib/python2.7/dist-packages/matplotlib/backends/__init__.py", line 32, in pylab_setup
    globals(),locals(),[backend_name],0)
  File "/usr/local/lib/python2.7/dist-packages/matplotlib/backends/backend_gtk3agg.py", line 11, in <module>
    from . import backend_gtk3
  File "/usr/local/lib/python2.7/dist-packages/matplotlib/backends/backend_gtk3.py", line 58, in <module>
    cursors.MOVE          : Gdk.Cursor.new(Gdk.CursorType.FLEUR),
TypeError: constructor returned NULL

而我在树莓派gui系统中运行上述脚本是可以生成test.png文件的。
所以上述错误只是出现在无gui的情况下，google了多个结果都没有解决方法。

1. 使用python绘制统计图库（类似matplotlib)可以在无gui的情况下根据所给数据生成统计图（png,jpeg等等)。

2. 或者有别的方法实现我上述需求。

Answer 1

import matplotlib
#matplotlib.use("Agg")
matplotlib.use("Pdf")
import matplotlib.pyplot as plt
import numpy as np

原因我在官网找到了下面这句话，解释了matplotlib.use("Agg")放在import matplotlib.pyplot as plt之前的原因。感谢@sPeng.使用matplotlib.use("Pdf")同样可以解决这个错误.

If you use the use() function, this must be done before importing matplotlib.pyplot. Calling use() after pyplot has been imported will have no effect. Using use() will require changes in your code if users want to use a different backend. Therefore, you should avoid explicitly calling use() unless absolutely necessary.

Answer 2

我也遇到同样的错误，我在raspberry pi 2上运行。为什么我添加了这句以后，反而没有图形输出了呢?

#matplotlib.use("Agg")
matplotlib.use("Pdf")