ValueError:无法将字符串转换为浮点:在python上绘制图形

ValueError: could not convert string to float .这就是我所做的:

import matplotlib.pyplot as plt
import csv
x = []
y = []
with open('DNSdata.csv', 'r') as csvfile:
    plots = csv.reader(csvfile, delimiter=',')
    for row in plots:
        x.append(float(row[0]))
        y.append(float(row[1]))
plt.plot(x, y, label='DNSdata')
plt.xlabel('x')
plt.ylabel('y')
plt.title('DNSdata')
plt.show()