#!/usr/bin/env python

# Plot Heat index graph with Matplotlib

from pylab import *

figure(figsize = (10, 9))
tt = range(20, 51)

def heat_index(t, r):
    "Calculate Heat index for T (°C) and RH (%)"

    c1 = -8.78469475556
    c2 = 1.61139411
    c3 = 2.33854883889
    c4 = -.14611605
    c5 = -.012308094
    c6 = -.0164248277778
    c7 = 2.211732e-3
    c8 = 7.2546e-4
    c9 = -3.582e-6

    hi = c1 + c2*t + c3*r + c4*t*r + c5*t*t + c6*r*r + c7*t*t*r + c8*t*r*r + c9*t*t*r*r
    return round(hi)

for r in range(100, 30, -10):
    plot(tt, [heat_index(t, r) for t in tt], label = "RH = %u%%" % r,
        lw = 1.5, ls = "dashed", marker = "o")

legend()
fill_between(tt, 0, 26.5,    color = "#0f0",    alpha = .5)
fill_between(tt, 26.5, 31.5, color = "#ff6",    alpha = .5)
fill_between(tt, 31.5, 40.5, color = "#ffd700", alpha = .5)
fill_between(tt, 40.5, 53.5, color = "#ff8c00", alpha = .5)
fill_between(tt, 53.5, 200,  color = "#f00",    alpha = .4)

xticks(range(100))
yticks(range(150))
axis([27, 45, 27, 60])
xlabel("T [°C]")
ylabel("Heat index")
title("Heat index based on temperature and relative humidity")
grid()
savefig("heat_index_plot.svg")

show()