Re: [Matplotlib-users] Controlling zorder of individual elements of the same type...

[John H. <jdh...@ac...>]

>>>>> "Fernando" == Fernando Perez <Fer...@co...> writes:

    Fernando> Hi all, I am having a bit of a problem with the order
    Fernando> things get drawn.  I looked at the zorder example and
    Fernando> the docs, but I can't seem to find a solution.  Here's
    Fernando> an illustration:

    Fernando> http://amath.colorado.edu/faculty/fperez/tmp/zorder_prob.png

    Fernando> The red wiggly line is drawn first, by a loglog() call,
    Fernando> and then the green one is an axhline() call.  However,
    Fernando> the green line ends up below the red one.  It turns out
    Fernando> that in cases where the range of the red stuff is above
    Fernando> the green cutoff, this causes the green line to be
    Fernando> totally obscured.  And for my plots, it's important that
    Fernando> the green line is clearly visible always.

    Fernando> My naive expectation was that whatever was called last
    Fernando> on the plot would end up on top, but that doesn't seem
    Fernando> to be the case.  I did a few experiments:

    Fernando> x=frange(0,2*pi,npts=100) figure()
    Fernando> plot(x,sin(x),x,cos(x),linewidth=10)
    Fernando> plot(x,sin(2*x),linewidth=10) axhline(0,linewidth=10)

Hmm, seems to work for me -- you didn't specify the colors in your
example which makes it hard to test, so I'll add them

    from pylab import *
    x=frange(0,2*pi,npts=100)
    figure()
    plot(x,sin(x),x,cos(x),linewidth=10, color='red')
    plot(x,sin(2*x),linewidth=10, color='green')
    axhline(0,linewidth=10, color='blue')
    show()

with results at http://matplotlib.sf.net/jdh.png

    Fernando> And I can't really figure out what determines the zorder
    Fernando> of all the line objects.

Well the zorder kwarg is your friend -- did you see
examples/zorder_demo.py ?  Eg

  plot(x,y, zorder=100)  # I'm on top!

I think I have an idea of what may be causing the plot order problem
you are experiencing

From axes.py draw method

        artists = []
        artists.extend(self.collections)
        artists.extend(self.patches)
        artists.extend(self.lines)
        artists.extend(self.texts)

        dsu = [ (a.zorder, a) for a in artists]
        dsu.sort()
        
        for zorder, a in dsu:
            a.draw(renderer)

Now, if I recall correctly, python sort doesn't guarantee preserving
order for equal element.  Since the order in the respective list
(patches, lines, etc) *is* determined by the order of the plot
commands, we might be better off with 

        dsu = [ (a.zorder, i, a) for i, a in enumerate(artists)]
        dsu.sort()
        
        for zorder, i, a in dsu:
            a.draw(renderer)

to guarantee relative order for artists with the same zorder.

Or is sort relative order preserving for equal elements?


JDH