title: Set subtitles font size on facet charts in Altair
body: <p>How can I edit the font size of the years appearing above each subgraph in the <a href="https://altair-viz.github.io/gallery/us_population_over_time_facet.html" rel="nofollow noreferrer">example</a> below:
  Take from the altair gallery</p>
  
  <pre><code>import altair as alt
  from vega_datasets import data
  
  source = data.population.url
  
  alt.Chart(source).mark_area().encode(
      x='age:O',
      y=alt.Y(
          'sum(people):Q',
          title='Population',
          axis=alt.Axis(format='~s')
      ),
      facet=alt.Facet('year:O', columns=5),
  ).properties(
      title='US Age Distribution By Year',
      width=90,
      height=80
  )
  </code></pre>