def build_xaxis(num, radius_increment=0.05):
  """Calculate the radius/diameter for the x-axis in plots."""
  x_axis = {
      'radius': [],
      'diameter': []
  }

  radius = 0
  for _ in range(num):
    radius = round(radius, 2)
    diameter = round(radius * 2, 1)
    x_axis['radius'].append(radius)
    x_axis['diameter'].append(diameter)
    radius += radius_increment

  return x_axis