title: creating quarterly `xts` time-series object for use with plot.xts
body: <p>I'm working to transform some data to a quarterly time <code>xts</code> series object. First my data wasn't <em>an appropriate time-based object</em> and now <code>as.yearqtr</code> is behaving in I cannot make sense of.</p>
  
  <p>I want the object <code>df</code> to be transformed so that I can plot it with <code>plot.xts</code>, my final goal, but I am stuck as shown below. </p>
  
  <pre><code>df &lt;- structure(list(yrQ = structure(1:7, .Label = c("2016-1", "2016-2", 
  "2016-3", "2016-4", "2016-5", "2016-6", "2016-7"), class = "factor"), 
      a = c(4.14, 2.83, 3.71, 4.15, 4.63, 4.91, 5.31), b = c(4.25, 
      3.5, 3.5, 3.5, 3.5, 3.5, 5)), .Names = c("yrQ", "a", "b"
  ), row.names = c(NA, 7L), class = "data.frame")
  df
  #      yrQ    a    b
  # 1 2016-1 4.14 4.25
  # 2 2016-2 2.83 3.21
  # 3 2016-3 3.71 3.21
  # 4 2016-4 4.15 3.21
  # 5 2016-5 4.63 3.21
  # 6 2016-6 4.91 3.21
  # 7 2016-7 5.31 5.00
  
  # install.packages(c("xts"), dependencies = TRUE)
  library(xts)
  xts(df, order.by = df[,1])
  # Error in xts(df, order.by = df[, 1]) : 
  #   order.by requires an appropriate time-based object
  
  df$yrQ &lt;- as.yearqtr(df$yrQ)
  df    
  #       yrQ    a    b
  # 1 2016 Q1 4.14 4.25
  # 2 2016 Q2 2.83 3.21
  # 3 2016 Q3 3.71 3.21
  # 4 2016 Q4 4.15 3.21
  # 5  NA QNA 4.63 3.21
  # 6  NA QNA 4.91 3.21
  # 7  NA QNA 5.31 5.00
  </code></pre>