title: Increasing bar width in bar chart using Altair
body: <p>I have the following dataframe and want to use <code>altair</code> library in Python to plot my bar chart. However, I couldn't figure out how to expand the width of each bar (same as <code>width=0.5</code> parameter in <code>matplotlib</code>).</p>
  
  <pre><code>import pandas as pd 
  from altair import * 
  
  ls = [[   1,  734], [   2, 1705], [   3, 2309],
        [   4, 2404], [   5, 2022], [   6, 1538],
        [   7, 1095], [   8,  770], [   9,  485],
        [  10,  312], [  11,  237], [  12,  153],
        [  13,  103], [  14,   69], [  15,   47],
        [  16,   39], [  17,   43], [  18,   28],
        [  19,   18]]
  df = pd.DataFrame(ls, columns=['n_authors', 'n_posters'])
  </code></pre>
  
  <p>Here is plotting function using <code>altair</code></p>
  
  <pre><code>bar_chart = Chart(df).mark_bar().encode(
      x=X('n_authors', title='Number of Authors in a Poster'), 
      y=Y('n_posters', title='Number of Posters'),
  ).configure_facet_cell(
      strokeWidth=1.0,
      height=200,
      width=300
   )
  bar_chart.display()
  </code></pre>
  
  <p>The plot looks like below from the script: </p>
  
  <p><img src="https://i.imgur.com/7IWuFMY.png" alt="Imgur"></p>