Idea for adding _normalize_data() back.

_data.py

def _normalize_data(chart):
    """Converts the data to a Pandas dataframe. Originally Nabarun's code (PR #5).

    Parameters
    ----------
    chart : altair.Chart
        The Altair chart object
    """
    spec = chart.to_dict()
    if not spec['data']:
        raise ValidationError('Please specify a data source.')

    if spec['data'].get('url'):
        df = pd.DataFrame(_fetch(spec['data']['url']))
    elif spec['data'].get('values'):
        return  # It's already a dataframe
    else:
        raise NotImplementedError('Given data specification is unsupported at the moment.')

    chart.data = df

test_data.py

import pytest
import altair as alt
import pandas as pd
from vega_datasets import data
from .._data import _normalize_data
from .._exceptions import ValidationError

def test_data_list():
    chart = alt.Chart(pd.DataFrame({'a': [1], 'b': [2], 'c': [3]})).mark_point()
    _normalize_data(chart)
    assert type(chart.data) == pd.DataFrame

def test_data_url():
    chart = alt.Chart(data.cars.url).mark_point()
    _normalize_data(chart)
    assert type(chart.data) == pd.DataFrame