A depiction of the electromagnetic spectrum

import numpy as np
import matplotlib.pyplot as plt

# wavelength range, nm
lmin, lmax = 250, 1000
x = np.linspace(lmin, lmax, 1000)
# A wave with a smoothly increasing wavelength
wv = (np.sin(10 * np.pi * x / (lmax+lmin-x)))[::-1]

fig = plt.figure()
ax = fig.add_subplot(111, facecolor='k')
ax.plot(x, wv, c='w', lw=2)
ax.set_xlim(250,1000)
ax.set_ylim(-2,2)

# Label and delimit the different regions of the electromagnetic spectrum
ax.text(310, 1.5, 'UV', color='w', fontdict={'fontsize': 20})
ax.text(530, 1.5, 'Visible', color='k', fontdict={'fontsize': 20})
ax.annotate('', (400, 1.3), (750, 1.3), arrowprops={'arrowstyle': '<|-|>',
                                                    'color': 'w', 'lw': 2})
ax.text(860, 1.5, 'IR', color='w', fontdict={'fontsize': 20})
ax.axvline(400, -2, 2, c='w', ls='--')
ax.axvline(750, -2, 2, c='w', ls='--')
# Horizontal "axis" across the centre of the wave
ax.axhline(c='w')
# Ditch the y-axis ticks and labels; label the x-axis
ax.yaxis.set_visible(False)
ax.set_xlabel(r'$\lambda\;/\mathrm{nm}$')

# Finally, add some colourful rectangles representing a rainbow in the
# visible region of the spectrum.
# Dictionary mapping of wavelength regions (nm) to approximate RGB values
rainbow_rgb = { (400, 440): '#8b00ff', (440, 460): '#4b0082',
                (460, 500): '#0000ff', (500, 570): '#00ff00',
                (570, 590): '#ffff00', (590, 620): '#ff7f00',
                (620, 750): '#ff0000'}
for wv_range, rgb in rainbow_rgb.items():
    ax.axvspan(*wv_range, color=rgb, ec='none', alpha=1)
plt.show()