Scatterplot Marker Size proportional to axis value

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:

434.0 448.0

Can anybody explain why they are not equal?

I'm not sure if it's relevant, but I'm using Python 2.7.12 and matplotlib 1.5.1