plot: Align doesn't exactly align axes

[sbinet]

as can be noticed from the produced plot in #732, (even with the 1:1 ratio), axes are not exactly aligned between plots (Y-axis for top-left/bottom-left, X-axis for bottom-left/bottom-right):

I've tested that with release up to v0.7.0 (so it's not a fall out from the "fonts revamp" work.)

[sbinet]

ha! it would seem the misalignment is coming from the glyphs being used for the scatter plot.
playing with the circle radius:

so it's probably some interplay between plot.Plot.DataCanvas and plot.GlyphBoxer
@ctessum any insight?