code: def get_pt(self, viewer, points, pt, canvas_radius=None):
          
          if canvas_radius is None:
              canvas_radius = self.cap_radius
  
          if hasattr(self, 'rot_deg'):
              # rotate point back to cartesian alignment for test
              ctr_pt = self.get_center_pt()
              pt = trcalc.rotate_coord(pt, [-self.rot_deg], ctr_pt)
  
          res = self.within_radius(viewer, points, pt, canvas_radius)
          return np.flatnonzero(res)
docs: Takes an array of points `points` and a target point `pt`.
          Returns the first index of the point that is within the
          radius of the target point.  If none of the points are within
          the radius, returns None.