title: Size parameter in Plotly R scatter plot with data and value does not work
body: <p>I have a data frame like this:</p>
  
  <pre><code>dput(df)
  structure(list(date = structure(c(16476, 16477, 16524, 16541, 
  16595, 16658, 16769, 16804, 16819, 16819, 16828, 16833, 16875
  ), class = "Date"), y = c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
  1L, 1L, 1L, 1L)), .Names = c("date", "y"), row.names = c(NA, 
  -13L), class = c("tbl_df", "tbl", "data.frame"))
  </code></pre>
  
  <p>I use plotly R to plot a scatter plot like this:</p>
  
  <pre><code>plot_ly(df, x = date, y = y, mode = 'markers')
  </code></pre>
  
  <p>The above statement works perfectly fine.</p>
  
  <p>Now, I just want to increase the size of the points in the scatter plot. Trying this messes up the whole plot including the x axis date scale:</p>
  
  <pre><code>plot_ly(df, x = date, y = y, mode = 'markers', size = 10)
  </code></pre>
  
  <p>Any idea what is going on? My version is:</p>
  
  <pre><code>packageVersion('plotly')
  [1] ‘3.5.5’
  </code></pre>