new_contents: """
  Polynomial Fit Plot with Regression Transform
  =============================================
  This example shows how to overlay data with multiple fitted polynomials using
  the regression transform.
  """
  # category: scatter plots
  
  import numpy as np
  import pandas as pd
  import altair as alt
  
  # Generate some random data
  rng = np.random.RandomState(1)
  x = rng.rand(40) ** 2
  y = 10 - 1. / (x + 0.1) + rng.randn(40)
  source = pd.DataFrame({'x': x, 'y': y})
  
  # Define the degree and color of the polynomial fits
  degree_list = [1, 3, 5]
  color_list = ['#5276A7', '#F18727', '#E0575A']
  
  base = alt.Chart(source).mark_circle(color='black').encode(
      alt.X('x'), alt.Y('y')
  )
  
  polynomial_fit = (base.transform_regression('x', 'y', method='poly', order=order)
                    .mark_line(color=color) 
                    for order, color in zip(degree_list, color_list))
  
  alt.layer(base, *polynomial_fit)
  
subject: add poly fit example with regression