[Tutor] Arrow of constant size on log plot

[Colin Ross]

I am attempting to draw an arrow that begins at a specified point on a
logarithmic plot and then extends a certain distance in the -Y direction.
This part is fine, however I would like to draw multiple arrows with the
same size corresponding to different points on a plot. If I specify a
certain value (as shown in the example below), the arrow size will change
depending on where the point is on the plot, since the X and Y axis are
logarithmic.

-------------------------------------------------------------

import numpy as np

import pylab as pl

x_1 = np.arange(10)

y_1 = np.arange(10)

x_1_avg = np.sum(x_1)/len(x_1)

y_1_avg = np.sum(y_1)/len(y_1)

x_2 = 3.

y_2 = 3.

pl.plot(x_1,y_1,'k')

pl.arrow(x_1_avg,y_1_avg , 0.0, -0.5, fc='b', ec='b', head_width=0.1,
head_length=0.1)

pl.arrow(x_2,y_2, 0.0, -0.5, fc='r', ec='r', head_width=0.1, head_length=0.1
)

pl.yscale('log')

pl.xscale('log')

pl.show()

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Any advice is appreciated.
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