title: ggplot - geom_line not plotting line
body: <p>with a dataframe <code>df</code> like below, Im trying to create a line chart for one of the parameters - <code>Income</code> - in the real data there are other parameters.</p>
  
  <pre><code>text &lt;- "
  Parameter,Company, Qtr, Value
  Income, Comp1, Q1FY18, 100
  Income, Comp1, Q2FY18, 110
  Income, Comp1, Q3FY18, 130
  Income, Comp2, Q1FY18, 60
  Income, Comp2, Q2FY18, 70
  Income, Comp2, Q3FY18, 90
  "
  df &lt;- read.table(textConnection(text), sep=",", header = T,
                   stringsAsFactors = F)
  </code></pre>
  
  <p>I created a function such that I can re-use this - the function takes a dataframe and set of metrics and plot them</p>
  
  <pre><code>plotMetricsLine &lt;- function(df, metrics) {
    df &lt;- df %&gt;%
      filter(Parameter %in% metrics)
    p &lt;- ggplot(data=df, aes(x=Qtr, y=Value,color = Company)) +
      geom_line(size=1)
    return(p)
  }
  </code></pre>
  
  <p>When I'm using that function by calling <code>plotMetricsLine(df, "Income")</code> - it is giving an empty plot as in the picture below. What is going on wrong here ?</p>
  
  <p><a href="https://i.stack.imgur.com/677Qq.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/677Qq.jpg" alt="enter image description here"></a></p>