title: Changing the layer shape for CPTPlotSpaceAnnotation?
body: <p>I want to add a widget to my graph that a user can click and drag. I've managed to do this using CPTPlotSpaceAnnonation (because I also want it to also scroll with the data) and CPTBorderedLayer. The basic functionality works and now I want to improve the look of the widget. So far it's just a green circle. </p>
  
  <p>The follow code generates the circle below, notice how the shadow is clipped to the rect of the layer and the border follows the layer and not the circle,</p>
  
  <p><img src="https://i.stack.imgur.com/7HkPl.png" alt="enter image description here"></p>
  
  <p>How can I stroke the circle and not the layer? I guess I can avoid clipping by making the frame of the layer larger. Is possible to use CAShapeLayer class with the Core Plot annotations? This would be an easy way of drawing the widgets.</p>
  
  <pre class="lang-c prettyprint-override"><code>// Add a circular annotation to graph with fill and shadow
  annotation = [[CPTPlotSpaceAnnotation alloc] 
                    initWithPlotSpace:graph.defaultPlotSpace 
                      anchorPlotPoint:anchorPoint];
  
  // Choosing line styles and fill for the widget
  NSRect frame = NSMakeRect(0, 0, 50.0, 50.0);
  CPTBorderedLayer *borderedLayer = [[CPTBorderedLayer alloc] initWithFrame:frame];
  
  // Circular shape with green fill
  CGPathRef outerPath = CGPathCreateWithEllipseInRect(frame, NULL);
  borderedLayer.outerBorderPath = outerPath;
  CPTFill *fill = [CPTFill fillWithColor:[CPTColor greenColor]];
  borderedLayer.fill = fill;
  
  // Basic shadow
  CPTMutableShadow *shadow = [CPTMutableShadow shadow];
  shadow.shadowOffset = CGSizeMake(0.0, 0.0);
  shadow.shadowBlurRadius = 6.0;
  shadow.shadowColor = [[CPTColor blackColor] colorWithAlphaComponent:1.0];
  borderedLayer.shadow = shadow;
  
  // Add to graph
  annotation.contentLayer = borderedLayer;
  annotation.contentLayer.opacity = 1.0;
  [graph addAnnotation:annotation];
  </code></pre>