title: How to toggle initial layer visibility in leaflet.js
body: <p>I've created a map in leaflet.js with multiple layers but can't work out how to toggle one layer to initially appear on the map.</p>
  
  <p><a href="http://www.stokesentinel.co.uk/Crime-map-hotspots-Staffordshire/story-24584825-detail/story.html" rel="noreferrer">Here is the link</a></p>
  
  <p>I'm sure it's fairly straightforward but just can't figure it out.</p>