title: ValueError when using Basemap.scatter()
body: <p>I have a dataframe with a bunch of time series geographical data that I am mapping.</p>
  
  <pre><code>my_map = Basemap(
  projection="tmerc",
  lon_0=-105,
  lat_0=39,
  ellps="WGS84",
  llcrnrlon=coords[0] - extra + 0.01 * w,
  llcrnrlat=coords[1] - extra + 0.01 * h,
  urcrnrlon=coords[2] + extra + 0.03 * w,
  urcrnrlat=coords[3] + extra + 0.01 * h,
  lat_ts=0,
  resolution="h",
  suppress_ticks=True)
  
  my_map.readshapefile("...", "colorado")
  
  df_map = pd.DataFrame({
      "poly": [Polygon(xy) for xy in my_map.colorado],
      "business_name": [business["NAME"] for business in my_map.colorado_info]
  })
  
  map_points = pd.Series([Point(my_map(mapped_x, mapped_y)) for mapped_x, mapped_y in zip(df[1], df[0])])
  business_points = MultiPoint(list(map_points.values))
  county_polygons = prep(MultiPolygon(list(df_map["poly"].values)))
  colorado_points = filter(county_polygons.contains, business_points)
  
  fig, ax = plt.subplots()
  map_colors = [date2num(i) for i in df["datecreated"]]
  dev = my_map.scatter(
      [geom.x for geom in colorado_points],
      [geom.y for geom in colorado_points],
      5, marker="o", lw=.25,
      c=map_colors, edgecolor='w', cmap=plt.cm.viridis,
      alpha=0.9, antialiased=True,
      label='Business Locations')
  </code></pre>
  
  <p>When I go to call <code>my_map.scatter()</code>, it keeps throwing this error:</p>
  
  <blockquote>
    <p>ValueError: to_rgba: Invalid rgba arg "689576.0" to_rgb: Invalid rgb
    arg "689576.0" cannot convert argument to rgb sequence</p>
  </blockquote>
  
  <p>My dataset has roughly 1.3M data points, with multiple, repeated dates correlating to different events. I would it wouldn't be a problem to use a colorbar for that many dates, seeing as there are 16M colors. <a href="http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.scatter" rel="nofollow noreferrer">Looking at the docs</a>, <code>matplotlib.pyplot.scatter()</code>, the <code>c</code> arg can be "a single color format string, or a sequence of color specifications of length N, or a sequence of N numbers to be mapped to colors".</p>
  
  <p>What am I doing wrong and how do I fix it?</p>
  
  <p>Full Traceback: <a href="https://paste.ubuntu.com/23813647/" rel="nofollow noreferrer">https://paste.ubuntu.com/23813647/</a></p>
  
  <p><a href="http://pastebin.com/zDpAC6Tb" rel="nofollow noreferrer">Minimal, Complete, and Verifiable example</a>.</p>