title: drawing predict line on an R graph
body: <p>I have a time series data:</p>
  
  <p>output of x data frame:</p>
  
  <pre><code>Date  vol
  1990  12
  1991  13
  1992  15
  1994  18
  1995  20
  1996  35
  </code></pre>
  
  <p>I am trying to plot this data and predict 4 year ahead as below:</p>
  
  <pre><code>plot(x$Date, x$vol, col="blue")
  x.lm&lt;-lm(x$Vol ~ x$Date)
  x.pre&lt;-predict(x.lm, n.ahead=4)
  abline(x.pre, col="red")
  </code></pre>
  
  <p>I get this error:</p>
  
  <pre><code>Error in int_abline(a = a, b = b, h = h, v = v, untf = untf, ...) : 
    invalid a=, b= specification
  </code></pre>
  
  <p>can anybody tell me what I am doing wrong?</p>