title: Converting pandas dataframe to numeric; seaborn can't plot
body: <p>I'm trying to create some charts using weather data, pandas, and seaborn. I'm having trouble using lmplot (or any other seaborn plot function for that matter), though. I'm being told it can't concatenate str and float objects, but I used convert_objects(convert_numeric=True) beforehand, so I'm not sure what the issue is, and when I just print the dataframe I don't see anything wrong, per se.</p>
  
  <pre><code>import numpy as np
  import pandas as pd
  import seaborn as sns
  new.convert_objects(convert_numeric=True)
  sns.lmplot("AvgSpeed", "Max5Speed", new)
  </code></pre>
  
  <p>Some of the examples of unwanted placeholder characters that I saw in the few non-numeric spaces just glancing through the dataset were "M", " ", "-", "null", and some other random strings. Would any of these cause a problem for convert_objects? Does seaborn know to ignore NaN? I don't know what's wrong. Thanks for the help.</p>