title: Constant error when using the plot_time_series() function
body: <p>I am trying to plot a graph using the plot_time_series function and keep on getting the following error</p>
  <pre><code>&gt; Retail %&gt;%
  +   plot_time_series(Date, Profit, .interactive = F)
  Error: Problem with `mutate()` input `.value_smooth`.
  x No method for class character.
  i Input `.value_smooth` is `auto_smooth(...)`.
  </code></pre>
  <p>Any suggestions? I tried uninstalling the timetk package but that didn't work.</p>