title: Moving a UIView along a CGPath according to touch position
body: <p>I have a line graph I've drawn in Quartz, and a UIView 'bubble' that I'd like to ideally pop up when the user touches the single plot line, and moves their finger along it. The bubble displays some extra graph information.</p>
  
  <p>I'd like to 'attach' the UIView to the CGPath plot, but I'm having trouble conceptually figuring out the best way to do this. I know you can animate a view along a CGPath, but this doesn't seem to work for me, because the user needs to 'scrub' along the graph themselves with their finger rather than any automatic animation.</p>
  
  <p>Does anyone have any suggestions of a good approach?</p>