title: D3: Simple Interaction - Circle with a click
body: <p>I am learning D3. I know easy things like making a scatter plot and all. My next step is trying some simple interactive moves. For example, after I have an svg appended, axes and grids made, now I wish to make a circle by clicking a point within the svg canvas. I guess I will have to record coordinate of the clicked point and then append a circle with its cx nad cy, but how? How to record the coordinate?</p>
  
  <p>I appreciate you show me a tutorial, give a hint or best of all an example.</p>