title: Two time series -appear similar but plot differently
body: <p>I have merged two series to plot with ggplot2 but the second series plots as an area rather than a line.  My input is a tibble which suggests something about the data.
  I have merged to xts series into a tibble,  when I plot it, the first series shows as a line and the second shows as an area.  I cannot understand why.  The two series are similar in magnitude.  The top of the red area is about right for the second series.
  Here is summary of the input data frame</p>
  
  <pre><code>&gt; summary(us_cdn_xts)
       Index               us10y2y          cdn10y2y      
   Min.   :2004-01-01   Min.   :-0.190   Min.   :-0.1400  
   1st Qu.:2007-09-13   1st Qu.: 0.770   1st Qu.: 0.0000  
   Median :2011-05-26   Median : 1.460   Median : 0.6500  
   Mean   :2011-05-26   Mean   : 1.401   Mean   : 0.6949  
   3rd Qu.:2015-02-05   3rd Qu.: 2.120   3rd Qu.: 1.2200  
   Max.   :2018-10-18   Max.   : 2.910   Max.   : 2.3200  
                        NA's   :1700     NA's   :159      
  &gt; 
  </code></pre>
  
  <p>Code is below</p>
  
  <pre><code>&gt; plot_data&lt;-tidy(us_cdn_xts)
  &gt; print(plot_data)
  # A tibble: 10,810 x 3
     index      series  value
     &lt;date&gt;     &lt;chr&gt;   &lt;dbl&gt;
   1 2004-01-01 us10y2y NA   
   2 2004-01-02 us10y2y  2.44
   3 2004-01-03 us10y2y NA   
   4 2004-01-04 us10y2y NA   
   5 2004-01-05 us10y2y  2.46
   6 2004-01-06 us10y2y  2.45
   7 2004-01-07 us10y2y  2.43
   8 2004-01-08 us10y2y  2.42
   9 2004-01-09 us10y2y  2.43
  10 2004-01-10 us10y2y NA   
  # ... with 10,800 more rows
  &gt; library(ggthemes)
  &gt; term_plot&lt;-ggplot(plot_data,aes(x=index,y=value,colour=series))+
  + geom_line()
  &gt; ggsave(term_plot,file="term_plot.png")
  Saving 7 x 7 in image
  Warning message:
  Removed 3 rows containing missing values (geom_path). 
  &gt; 
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/GfOcq.png" alt="plot"></p>