Scatterplot Marker Size proportional to axis value - why is the number of pixels for the x and y axis different for aspect='equal'?

问题

My question is as far as I see closely related to this post.

I need to plot some data with marker size strictly proportional to the value of the axes. (Already asked the question here).

My approach is the following:

Create empty scatterplot for pixel reference
Scatter 2 points on the lower left and upper right corner
Limit the axes to exactly these two points
use transData.transform to get the pixel values of those two points
get the pixel distance as the difference in pixel number of these two points
(now that I have the distance-to-pixel-ratio, scatter my data with
s=(size*dist_to_pix_ratio)**2; but that is not important right now.)

Problem is: when I do exactly what I've described, I get two different values for the pixel number for the y-axis and the x-axis.

Here is a minimal code:

import matplotlib.pyplot as plt

fig = plt.figure(figsize=(7,7))
ax1 = fig.add_subplot(111, aspect='equal')

#setting up an empty scatterplot for pixel reference
xedges=[0.0, 1.0]
yedges=[0.0, 1.0]
emptyscatter=ax1.scatter(xedges, yedges, s=0.0)

#set axes limits
ax1.set_xlim(0.00,1.00)
ax1.set_ylim(0.00,1.00)   


# Calculating the ratio of pixel-to-unit
upright = ax1.transData.transform((1.0,1.0))
lowleft = ax1.transData.transform((0.0,0.0))
x_to_pix_ratio = upright[0] - lowleft[0]
y_to_pix_ratio = upright[1] - lowleft[1]

print x_to_pix_ratio, y_to_pix_ratio

which returns: