title: minimum value of y axis is not being applied in matplotlib vlines plot
body: <p>I am doing a vlines plot in matplotlib and I have all my y values in the dataset as <code>&gt;=0</code>. I want my y axis bottom most tick to read <code>0</code>, but instead, I get -500.</p>
  
  <p>Here is the code:</p>
  
  <pre><code>#!/usr/bin/env python
  
  import numpy as np
  from matplotlib import pyplot as plt, dates as mdates
  import datetime as dt, time
  
  # Read the data and turn it into a numpy array
  #store = map(lambda line: map(int, line.strip().split()), open(name + '.txt').readlines())
  store = [
      [1293606162197, 0, 0],
      [1293605477994, 63, 0],
      [1293605478057, 0, 0],
      [1293605478072, 2735, 1249],
      [1293606162213, 0, 0],
      [1293606162229, 0, 0],
  ]
  
  nstore = np.array(store)
  
  # Get arrays of each columns in the store array
  d = nstore[:,0]
  y1 = nstore[:,1]
  y2 = nstore[:,2]
  
  # Get arrays of values to be passed to matplotlib
  s = d / 1000
  dts = map(dt.datetime.fromtimestamp, s)
  fds = mdates.date2num(dts)
  
  # new figure and subplot
  fig = plt.figure()
  ax = fig.add_subplot(111)
  
  # Plot using vlines
  ax.vlines(fds, [0], y1, 'red')
  
  # set xaxis tick settings
  ax.xaxis.set_major_locator(mdates.MinuteLocator())
  ax.xaxis.set_major_formatter(mdates.DateFormatter('%m/%d %H:%M'))
  
  for label in ax.xaxis.get_ticklabels():
      label.set_rotation('vertical')
  
  fig.subplots_adjust(bottom=.25)
  
  # Set the y axis bottom limit to 0
  ax.set_ylim(bottom=0)   # &lt;&lt;- THIS DOES NOT SEEM TO BE WORKING
  
  # Save the plot figure
  fig.savefig('out.png')
  </code></pre>
  
  <p>and here is the plot I get:
  <img src="https://i.stack.imgur.com/3jHcy.png" alt="enter image description here"></p>
  
  <p>Can anyone point to me what I am doing wrong? Also, if you can point me to the docs that have the details I need, that would be great. Thanks.</p>
  
  <p><em>Question is a follow up of <a href="https://stackoverflow.com/questions/5498510/creating-graph-with-date-and-time-in-axis-labels-with-matplotlib">Creating graph with date and time in axis labels with matplotlib</a></em></p>