title: How to change d3 tickmark lines to circles
body: <p>How do I create a tickmark like this in D3 axis? </p>
  
  <p>--o---o---o---o---o</p>
  
  <p>instead of the default 
  ---|---|---|</p>
  
  <p>I searched the D3 examples, but could not see any sample of changing the tickmark from lines to circles.. </p>
  
  <p>Any help would be greatly appreciated..</p>