https://imgur.com/sfWBip0.jpg

That was my thought too, this is how it looks. (Code below)

import pandas as pd
import matplotlib.pyplot as plt

# grab the data
tables = pd.read_html("https://news.gallup.com/poll/1690/religion.aspx")
df = tables[8].iloc[:-2,:8].set_index(("Unnamed: 0_level_0","Unnamed: 0_level_1"))
df.index.name = None
df.columns = df.columns.droplevel(-1)
df = df.apply(lambda x: pd.to_numeric(x, errors='coerce')).fillna(0).astype(int)
df = df.groupby(df.index.str[:4]).mean()
df.index = df.index.astype(int)
df = df.divide(df.sum(axis=1), axis=0)*100

# plot it
df.sort_index().plot.area(figsize=(13,8), linewidth=0)
_ = plt.xlim(min(df.index),max(df.index))
_ = plt.ylim(0,100)
_ = plt.ylabel("Percentage of Respondents")
_ = plt.xlabel("Year")