Question

I am not able to draw a simple, vertical arrow in the following log-log plot:

#!/usr/bin/python2

import matplotlib.pyplot as plt
import matplotlib as mpl

plt.yscale('log')
plt.xscale('log')
plt.ylim((1e-20,1e-10))
plt.xlim((1e-12,1))

plt.arrow(0.00006666, 1e-20, 0, 1e-8 - 1e-20, length_includes_head=True)

plt.savefig('test.pdf')

It just doesn’t show. From the documentation it appears as if all the arguments, like width, height and so on relate to the scale of the axis. This is very counter-intuitive. I tried using twin() of the axisartist package to define an axis on top of mine with limits (0,1), (0,1) to have more control over the arrow’s parameters, but I couldn’t figure out how to have a completely independent axis on top of the primary one.

Any ideas?

Answer 1

Subplots approach

After creating the subplots do the following

• Align the positions
• Use set_axis_off() to turn the axis off (ticks, labels, etc)
• Draw the arrow!

So a few lines gets whats you want!

E.g.

#!/usr/bin/python2

import matplotlib.pyplot as plt

hax = plt.subplot(1,2,1)
plt.yscale('log')
plt.xscale('log')
plt.ylim((1e-20,1e-10))
plt.xlim((1e-12,1))

hax2 = plt.subplot(1,2,2)
plt.arrow(0.1, 1, 0, 1, length_includes_head=True)

hax.set_position([0.1, 0.1, 0.8, 0.8])
hax2.set_position([0.1, 0.1, 0.8, 0.8])

hax2.set_axis_off()

plt.savefig('test.pdf')

Rescale data

Alternatively a possibly easier approach, though the axis labels may be tricky, is to rescale the data.

i.e.

import numpy 

# Other import commands and data input

plt.plot(numpy.log10(x), numpy.log10(y))) 

Not a great solution, but a decent result if you can handle the tick labels!