title: position_fill requires non-overlapping x intervals in ggolot
body: <p>I am using ggplot to produce a bar chart stacked by a factor. And I use the following code</p>
  
  <pre><code>ggplot(X) +
    geom_bar(aes(X$Decade, fill=factor(X$Motivation)),
         position='fill') +
    theme(legend.position = "bottom")+
    theme(axis.title.x = element_text(size = 20),
      axis.title.y = element_text(size = 20))+
    theme(text = element_text(size=20))+
    theme(legend.text = element_text(size = 16),legend.title = element_blank())+
    xlab("Decade")+
    ylab("Percentage")+
    scale_y_continuous(labels=percent)
  </code></pre>
  
  <p>However the program gives the following warning,</p>
  
  <blockquote>
    <p>Warning message:
        position_fill requires non-overlapping x intervals </p>
  </blockquote>
  
  <p>The resulted figure is the below
  <img src="https://i.stack.imgur.com/kmLiW.png" alt="plot"></p>
  
  <p>It seems that the warning liers in the decade 1940. When the observations of 1940 are this <img src="https://i.stack.imgur.com/seQxI.png" alt="this"></p>
  
  <p>I don't know what's the problem here, since other observations look like those of 1940 and when I removed 1940 observations, no warnings any more.</p>