import numpy as np
import matplotlib.pyplot as plt

year, cases_inv, cases_noninv = np.loadtxt("wnv.txt", unpack=True, skiprows=4)
fig, ax = plt.subplots()

w = 0.3
noninv_bars = ax.bar(year - w, cases_noninv, w, color="gray")
inv_bars = ax.bar(year, cases_inv, w, color="k")

ax.set_ylabel("Number of cases")
ax.set_xlabel("Year")
ax.set_title(
    "Cases of neuroinvasive and non-neuroinvasive\n"
    "West Nile Virus disease in the United States, 1999-2008"
)
ax.set_xticks(year)
ax.legend((noninv_bars, inv_bars), ("Non-neuroinvasive", "Neuroinvasive"))

plt.show()