An annotated plot of a share price

Another example of an annotated plot, this time of the share price of BP plc (LSE: BP) with a couple of notable events added to it. The necessary data for this example can be downloaded from Yahoo! Finance.

import datetime
import numpy as np
import matplotlib.pyplot as plt
from matplotlib.dates import strpdate2num
from datetime import datetime

def date_to_int(s):
    epoch = datetime(year=1970, month=1, day=1)
    date = datetime.strptime(s, '%Y-%m-%d')
    return (date - epoch).days

def bindate_to_int(bs):
    return date_to_int(bs.decode('ascii'))

dt = np.dtype([('daynum','i8'), ('close', 'f8')])
share_price = np.loadtxt('bp-share-prices.csv', skiprows=1, delimiter=',',
                      usecols=(0,4), converters={0: bindate_to_int},
                      dtype=dt)
fig = plt.figure()
ax = fig.add_subplot(111)
ax.plot(share_price['daynum'], share_price['close'], c='g')
ax.fill_between(share_price['daynum'], 0, share_price['close'], facecolor='g',
                alpha=0.5)

daymin, daymax = share_price['daynum'].min(), share_price['daynum'].max()
ax.set_xlim(daymin, daymax)

price_max = share_price['close'].max()

def get_xy(date):
    """ Return the (x,y) coordinates of the share price on a given date. """
    x = date_to_int(date)
    return share_price[np.where(share_price['daynum']==x)][0]

# A horizontal arrow and label
x,y = get_xy('1999-10-01')
ax.annotate('Share split', (x,y), xytext = (x+1000,y), va='center',
            arrowprops=dict(facecolor='black', shrink=0.05))
# A vertical arrow and label
x,y = get_xy('2010-04-20')
ax.annotate('Deepwater Horizon\noil spill', (x,y), xytext = (x,price_max*0.9),
            arrowprops=dict(facecolor='black', shrink=0.05), ha='center')

years = range(1989,2015,2)
ax.set_xticks([date_to_int('{:4d}-01-01'.format(year)) for year in years])
ax.set_xticklabels(years, rotation=90)

plt.show()

We need to do some work to read in the date column: first decode the byte string read in from the file to ASCII (bindate_to_int), then use datetime (see Section 4.5.3 of the book) to convert it into an integer number of days since some reference date (epoch): here we choose the Unix epoch, 1 January 1970 (date_to_int).
`ax.fill_between_ fills the region below the plotted line with a single colour.
We rotate the year labels so there's enough room for them (reading bottom-to-top).