title: gganimate error with long-format data and plot_usmap
body: <p>I'm trying to animate a map graph using <code>gganimate</code> but am running into the following error:</p>
  <pre><code>Error in seq.default(range[1], range[2], length.out = nframes) : 
  'from' must be a finite number
  </code></pre>
  <p>It seems like this usually occurs when the variable used for <code>transition_time()</code> isn't numeric or the data's in the wrong form, but I don't know how that could be in my case; my data's in the proper long format and the variable for <code>transition_time()</code> is numeric and always finite.</p>
  <p>Is this a specific issue with <code>plot_usmap()</code>? Am I missing something obvious?</p>
  <p>My data is on <a href="https://github.com/burialrune/covid/blob/main/helpdata.Rda" rel="nofollow noreferrer">GitHub</a>. Code below:</p>
  <pre><code>library(&quot;usmap&quot;)
  library(&quot;ggplot2&quot;)
  library(&quot;gganimate&quot;)
  
  load(&quot;helpdata.Rda&quot;)
  
  check = plot_usmap(data = df_map, values = &quot;frac_cov&quot;) +
    scale_fill_stepsn(breaks=c(-100, 0, 0.1, 0.2, 0.3, 0.4, 0.5,
                               0.6, 0.7, 0.8, 0.9, 1, 100),
                      colors=c(&quot;red&quot;, &quot;gray100&quot;, &quot;gray90&quot;, &quot;gray80&quot;, &quot;gray70&quot;, &quot;gray60&quot;,
                               &quot;gray50&quot;, &quot;gray40&quot;, &quot;gray30&quot;, &quot;gray20&quot;, &quot;gray10&quot;, &quot;red&quot;),
                      na.value=&quot;white&quot;) +
    labs(fill = &quot;&quot;, title = &quot;Fraction of excess deaths\nattributed to COVID-19&quot;) +
    theme(legend.position = &quot;right&quot;, plot.title = element_text(hjust = 0.5)) +
    transition_time(month)
  
  anim &lt;- animate(check, nframes=9, fps=1)
  </code></pre>
  <p>Thanks so much!</p>