title: Matplotlib y axis values are not ordered
body: <p>I am trying to plot using matplotlib. The plot showed a problem that the Y axis is not ordered.</p>
  
  <p>Here is the code.</p>
  
  <pre><code># -*- coding: UTF-8 -*-
  import matplotlib.pyplot as plt
  import matplotlib.dates as mdates
  import datetime
  import numpy as np
  I020 = [ line.strip('\n').split(",") for line in 
  open(r'D:\Users\a0476\Anaconda3\TickData\PV5sdata1.csv')][1:]
  Time = [ datetime.datetime.strptime(line[0],"%H%M%S%f") for line in I020 ]
  Time1 = [ mdates.date2num(line) for line in Time ]
  Solar = [ line[1] for line in I020 ]
  order = np.argsort(Time1)
  xs = np.array(Time1)[order]
  ys = np.array(Solar)[order]
  plt.title('Solar data')
  plt.xlabel('Time')
  plt.ylabel('Solar')
  ax.plot_date(xs, ys, 'k-')
  hfmt = mdates.DateFormatter('%H:%M:%S')
  ax.xaxis.set_major_formatter(hfmt)
  plt.show()
  </code></pre>
  
  <p>The CSV data</p>
  
  <pre><code>time        solar
  7000000     50.35
  8000000     41.01
  9000000     69.16
  10000000    94.5
  11000000    111.9
  12000000    103
  13000000    98.6
  14000000    36.45
  15000000    34.74
  16000000    34.17
  17000000    34.6
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/7mC1K.png" alt="enter image description here"></p>