使用matplotlib向图形添加误差线

Question

所以现在，我正试图将错误栏添加到现有的图表中，但在运行代码时，我仍会遇到一些错误。下面是我添加的内容被注释掉时的代码（没有错误条）。从中提取信息的文件包含4列，其中第4列是垂直错误。当我运行包含注释行的代码时，会得到以下错误

Traceback (most recent call last):
File "39.py", line 37, in <module>
plot_graph()
File "39.py", line 29, in plot_graph
plt.errorbar(x1,y1, yerr = z1, marker = 'none', linestyle = 'none')
File "/Users/Bashe/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/matplotlib/pyplot.py", line 2697, in errorbar
errorevery=errorevery, capthick=capthick, **kwargs)
File "/Users/Bashe/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/matplotlib/axes.py", line 5758, in errorbar
in cbook.safezip(y, yerr)]
TypeError: unsupported operand type(s) for -: 'str' and 'str'

这是我的密码。希望有人能告诉我是什么导致了这个问题。

import os
import pylab as plt

def plot_graph():
    file='Graph.txt'
    x = []
    y = []
    #z = []
    x1 = []
    y1 = []
    #z1 = []
    t = []
    t1 = []
    for dirpath,dirs,files in os.walk('/Users/Bashe/Desktop/121210 p2/'):
        if file in files:
            infile = open(os.path.join(dirpath, "Graph.txt"), "r")
            for columns in (raw.strip().split() for raw in infile):
                t = columns[0]
                x = columns[1]          
                y = columns[2]
                #z = columns[3]
                x1.append(str(x))
                y1.append(str(y))
                #z1.append(str(z))
            t1.append(str(t))
            savepath = os.path.join(dirpath, 'Mean vs Temperature for %s.png' %(t1[0]))
            plt.plot(x1,y1, marker ='o', linestyle = '--')
            #plt.errorbar(x1,y1, yerr = z1, marker = 'none', linestyle = 'none')
            plt.xlabel('Temperature')
            plt.ylabel('Mean')
            plt.title('Mean vs Temperature for %s probe concentration' %(t1[0]))
            plt.savefig(savepath)
            #plt.show()
            infile.close()