 codex/past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2016/iii/paper-207-sir-states.py

"""SIR population-flow diagram; Python 3.14, matplotlib 3.10.7.
Run from the desired output directory; writes paper-207-sir-states.png there.
"""
import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt
from matplotlib.patches import FancyBboxPatch
fig,ax=plt.subplots(figsize=(7,2),dpi=120,facecolor='white')
ax.set(xlim=(0,1),ylim=(0,1));ax.axis('off')
for x,label in zip([.16,.5,.84],['Susceptible $S$','Infectious $I$','Recovered $R$']):
 ax.add_patch(FancyBboxPatch((x-.125,.25),.25,.32,boxstyle='round,pad=0.01',facecolor='#eef6ed',edgecolor='#356143',linewidth=1.5))
 ax.text(x,.41,label,ha='center',va='center',fontsize=11)
for x1,x2,label in [(.30,.36,r'$\beta S I$'),(.64,.70,r'$\gamma I$')]:
 ax.annotate('',xy=(x2,.41),xytext=(x1,.41),arrowprops={'arrowstyle':'->','lw':1.8,'color':'#356143'})
 ax.text((x1+x2)/2,.64,label,ha='center',fontsize=13)
ax.text(.5,.89,'SIR model with permanent immunity',ha='center',fontsize=13)
ax.text(.5,.09,r'Population flow rates; individual hazards are $\beta I$ and $\gamma$.',ha='center',fontsize=10)
fig.subplots_adjust(left=.01,right=.99,bottom=.03,top=.98)
fig.savefig('paper-207-sir-states.png',dpi=120,facecolor='white',transparent=False)
plt.close(fig)