title: could not find the function ggplot2
body: <p>after estensive research, I decided to post my question.
  I need to work on some graphics, so I decided to install ggplot2 package.</p>
  
  <p>Well what I did was:</p>
  
  <pre><code>install.packages("ggplot2")
  require(ggplot2)
  </code></pre>
  
  <p>But nothing happens, when I try to use ggplot it comes out something like "could not find the function "ggplot2"" as if the packages was never installed.</p>
  
  <p>i suppose it is something to do with directories in which the package is downloaded, but seriously, have no idea.</p>
  
  <p>I get the error using the code:</p>
  
  <pre><code>ggplot2(data=tweets, aes(tweets$created_at)) + 
  geom_bar(aes(fill=..count..), alpha=0.5, size=0.5, binwidth=60*5) + 
  scale_x_datetime("tweets$created_at") + 
  scale_y_continuous("Number of tweets")
  </code></pre>
  
  <p>any way I paste what it shows after the "installation":</p>
  
  <blockquote>
    <p>install.packages("ggplot2")
    --- Please select a CRAN mirror for use in this session ---
    provo con l'URL<br>
    '<a href="http://www.freestatistics.org/cran/bin/macosx/contrib/3.0/ggplot2_0.9.3.1.tgz" rel="nofollow">http://www.freestatistics.org/cran/bin/macosx/contrib/3.0/ggplot2_0.9.3.1.tgz</a>'
    Content type 'application/x-gzip' length 2647855 bytes (2.5 Mb)
    URL aperto</p>
  </blockquote>
  
  <pre><code>==================================================
  </code></pre>
  
  <blockquote>
    <p>downloaded 2.5 Mb</p>
    
    <p>The downloaded binary packages are in
        /var/folders/zr/t7y5pvy94n557yv7pv2hlx6r0000gn/T//Rtmp1mD2Vi/downloaded_packages</p>
  </blockquote>
  
  <p>Help will be appreciated"""
  cheeers
  vvvv</p>