title: Value Error with color array when slicing values for scatter plot
body: <p>I want to specify the frequency of markers that are printed in my scatter plot.
  After being unsuccessful with markevery (<a href="https://stackoverflow.com/questions/21761437/problems-with-markevery-attributeerror-unknown-property-markevery">other stackoverflow question: Problems with using markevery</a>) I followed the suggestion to slice my values using the notation of x[::5] and y[::5] for every 5th value. </p>
  
  <p>However, now I get a different error. That is,</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:\Users\mkupfer\NASA_SJSU_UARC_work\Info\CodingExamples\PythonExamples\X-Y-Value_Plot_Z-SimTime_02_noSectors.py", line 26, in &lt;module&gt;
      timePlot = ax.scatter(x[::5], y[::5], s=50, c=timeList, marker = marker.next(), edgecolors='none', norm=cNorm, cmap = plt.matplotlib.cm.jet) #cm.Spectral_r
    File "C:\Python27\lib\site-packages\matplotlib\axes.py", line 5715, in scatter
      colors = mcolors.colorConverter.to_rgba_array(c, alpha)
    File "C:\Python27\lib\site-packages\matplotlib\colors.py", line 380, in to_rgba_array
      raise ValueError("Color array must be two-dimensional")
  ValueError: Color array must be two-dimensional
  </code></pre>
  
  <p>Here is a simplified version of my code:</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  import matplotlib.colors as colors
  import matplotlib.lines as lines
  from matplotlib import cm
  import csv
  import itertools
  import random
  
  #callSignList = [AMF2052,AMF2052,AMF2052,AMF2052,AMF2052]
  xList = random.sample(xrange(100), 100)
  yList = random.sample(xrange(100), 100)
  timeList = random.sample(xrange(100), 100)
  
  #prepare the plot
  fig = plt.figure(figsize=(18,13))
  ax = fig.add_subplot(111)
  
  cNorm  = plt.matplotlib.colors.Normalize(vmin=0, vmax=3600)
  marker = itertools.cycle(('o', '^', '+', '8', 's', 'p', 'x'))
  
  x = xList
  y = yList
  
  timePlot = ax.scatter(x[::5], y[::5], s=50, c=timeList, marker = marker.next(), edgecolors='none', norm=cNorm, cmap = plt.matplotlib.cm.jet) #cm.Spectral_r
  fig.subplots_adjust(top=0.90, bottom=0.15, hspace=0.25,)
  
  # Now adding the colorbar
  #fig.colorbar(timePlot, shrink=0.5, aspect=10, orientation="horizontal")
  cax = fig.add_axes([0.15, 0.06, 0.7, 0.05])
  #The numbers in the square brackets of add_axes refer to [left, bottom, width, height],
  #where the coordinates are just fractions that go from 0 to 1 of the plotting area.
  
  #ax.colorbar(timePlot)
  cbar = fig.colorbar(timePlot, cax, orientation='horizontal')
  cbar.set_label('Relative Simulation Time')
  
  plt.show()
  </code></pre>
  
  <p>Can someone please give me an idea where I made a mistake?
  Any help is appreciated. Thanks.</p>