title: Error in grDevices::png
body: <pre><code>quick.hist&lt;-ggplot(ufo.us, aes(X=DateOccured))+geom_histogram()+scale_x_date()
  ggsave(plot=quick.hist,filename= "../images/hist.png",height=6, width=8)
  </code></pre>
  
  <p>file format is simple:</p>
  
  <p>it contains 2 dates,city,state,duration</p>
  
  <p>I am getting error during plot:that is during <code>ggsave()</code></p>
  
  <pre><code>Error in grDevices::png(..., width = width, height = height, res = dpi,  
    unable to start png() device
  </code></pre>
  
  <p>In addition: Warning messages:</p>
  
  <pre><code> In grDevices::png(..., width = width, height = height, res = dpi,  
    unable to open file '../images/hist.png' for writing
  
   In grDevices::png(..., width = width, height = height, res = dpi,  
    opening device failed
  </code></pre>
  
  <p>I am very new to R and getting error. I have just started copying the book code for histogram chart, first chapter from book machine learning hacker perspective.</p>