title: Can't reproduce stat_smooth using `loess` when x-axis is Date
body: <p>I am using <code>ggplot2</code> to get a smoothed estimation of my data</p>
  
  <pre><code>ggplot(yy)+geom_smooth(aes(x=Date,y=value),method='loess')
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/5oApp.png" alt="enter image description here"></p>
  
  <p>It works fine. Now, when try to reproduce this using <code>loess</code> function directly, I get an error:</p>
  
  <pre><code>loess(value~Date,yy)
  Error in simpleLoess(y, x, w, span, degree, parametric, drop.square, normalize,  : 
    NA/NaN/Inf in foreign function call (arg 2)
  In addition: Warning message:
  In simpleLoess(y, x, w, span, degree, parametric, drop.square, normalize,  :
    NAs introduced by coercion
  </code></pre>
  
  <p>I think that <code>ggplot2</code> version works because it sets some default parameters. </p>
  
  <p>Here my data yy :</p>
  
  <pre><code>dput(yy)
  structure(list(Date = structure(c(9862, 9893, 9921, 9982, 10043, 
  10105, 10135, 10196, 10227, 10258, 10317, 10408, 10470, 11778, 
  12753, 13118, 13483, 13848, 14518, 10013, 10166, 11747, 11869, 
  11931, 12053, 12173, 12234, 12296, 12357, 12418, 12509, 12631, 
  12662, 12723, 12904, 12996, 13088, 13545, 13665, 13757, 13818, 
  14304, 14457, 10682, 10712, 10743, 10773, 13910, 14031, 14184, 
  13970, 10286, 10347, 10378, 10439, 10500, 10531, 10561, 10592, 
  10623, 10651, 10804, 10835, 10865, 10896, 11900, 12539, 12570, 
  12600, 12692, 11017, 11048, 11078, 11109, 11139, 11170, 11201, 
  11231, 11262, 11354, 11413, 11443, 11474, 11504, 11535, 11566, 
  11627, 11688, 11839, 12022, 12204, 12843, 11382, 11808, 11992, 
  12326, 12387, 13361, 13634, 14000, 14365, 11596, 11657, 11719, 
  11961, 12084, 12112, 12143, 12265, 12784, 13149, 13726, 14123, 
  14549, 14396), class = "Date"), value = c(31.8333333333333, 38.2, 
  28.8333333333333, 29.1666666666667, 50.6, 28.8333333333333, 34.6, 
  34.4, 34.4, 35.6, 79.1666666666667, 96.5714285714286, 124, 29.2, 
  8, 10, 66.6, 10, 20.25, 23.75, 49, 93, 49.7142857142857, 40.5, 
  73.8, 55, 71.2, 32.6, 27.25, 27.5, 24.75, 30.2, 21.4, 16.2, 27.5, 
  26.75, 18.75, 25.3333333333333, 39.8, 43.25, 22.6666666666667, 
  54.6666666666667, 67, 112, 91.5, 93, 106.666666666667, 91, 48.6666666666667, 
  50, 68, 300, 280, 290, 200, 301, 303, 300, 200, 150, 200, 200, 
  200, 100, 100, 63.3333333333333, 2, 3, 2, 2, 173, 169, 62, 130, 
  108, 154, 65, 94, 68, 220.5, 223, 210, 211, 214, 202, 212.333333333333, 
  193, 198.666666666667, 112.5, 133.666666666667, 106, 216.5, 206, 
  121.5, 39, 87, 93, 118, 115, 206, 95, 213, 90, 84, 85, 54, 34, 
  48, 52, 218, 187, 114, 96, 114, 42)), row.names = c(NA, -115L
  ), class = c("data.table", "data.frame"), .Names = c("Date", 
  "value"), .internal.selfref = &lt;pointer: 0x0000000000230788&gt;)
  </code></pre>