Translate from user data coordinate system to display coordinate system.
and use edgecolors=”none” to plot faces with no outlines.
import numpy as np
fig = figure()
ax = fig.add_subplot(111)
dx_in_points = np.diff(ax.transData.transform(zip([0]*len(dx), dx)))
scatter(x,y,c=z,s=dx_in_points**2,marker="s", edgecolors="none")