"""
Candlestick Chart
=================
A candlestick chart inspired from `Protovis <http://mbostock.github.io/protovis/ex/candlestick.html>`_. 
This example shows the performance of the Chicago Board Options Exchange `Volatility Index <https://en.wikipedia.org/wiki/VIX>`_ (VIX) 
in the summer of 2009. The thick bar represents the opening and closing prices, 
while the thin bar shows intraday high and low prices; if the index closed higher on a given day, the bars are colored green rather than red.
"""
# category: advanced calculations
import altair as alt
from vega_datasets import data

source = data.ohlc()

open_close_color = alt.condition(
    "datum.open <= datum.close",
    alt.value("#06982d"),
    alt.value("#ae1325")
)

base = alt.Chart(source).encode(
    alt.X('date:T')
        .axis(format='%m/%d', labelAngle=-45)
        .title('Date in 2009'),
    color=open_close_color
)

rule = base.mark_rule().encode(
    alt.Y('low:Q')
        .title('Price')
        .scale(zero=False),
    alt.Y2('high:Q')
)

bar = base.mark_bar().encode(
    alt.Y('open:Q'),
    alt.Y2('close:Q')
)

rule + bar