change figure size for boxplot
plt.figure(figsize=(150, 50))
df['IncorporationDate'].value_counts().sort_index().ix[1900:].plot(kind='bar')
plt.xticks(size=80)
plt.title('Incorporation Date', size=80)
plt.xlabel('Year', size=80)
plt.ylabel('Count', size=80)
plt.show()