Fix broken scatter/fill_between in log-scale axis

[lucasb-eyer]

I did try to fix this myself, but failed. So I asked Codex (gpt5-high) and it found the issue and proposed this fix, which I verified works, and I streamlined its code a bit.

Here's what it had to say:

• mplexporter/mplexporter/exporter.py:6-275 now imports numpy/mpath and routes collections through _prepare_collection_data, which preserves data-space vertices/offsets whenever a non-affine ax.transData branch is
  involved (log scales, semilog, etc.). As a result, fill_between, scatter, and other PathCollections on log axes export with pathcoordinates/offsetcoordinates == 'data', so the rendered polygons obey zooming/panning.
• mpld3/tests/test_elements.py:76-103 adds regression tests for both fill_between and scatter on logarithmic axes to guard against future regressions in the exported coordinate system.

Here's an example of my app before this fix:

And after this fix:

[lucasb-eyer]

PS: this is the test it's talking about adding to mpld3 repo. I would open a PR there which also moves the submodule, if you agree this is meaningful:

diff --git a/mpld3/tests/test_elements.py b/mpld3/tests/test_elements.py
index 8a89772..dbce6bf 100644
--- a/mpld3/tests/test_elements.py
+++ b/mpld3/tests/test_elements.py
@@ -76,6 +76,29 @@ def test_scatter():
                  [[7.607257743127308, 0.0, 0.0, 7.607257743127308, 0.0, 0.0]])
 
 
+def test_fill_between_logscale():
+    fig, ax = plt.subplots()
+    ax.set_xscale('log')
+    ax.set_yscale('log')
+    x = np.logspace(0, 2, 5)
+    ax.fill_between(x, x, x * 2)
+    rep = fig_to_dict(fig)
+    coll = rep['axes'][0]['collections'][0]
+
+    assert_equal(coll['pathcoordinates'], 'data')
+
+
+def test_scatter_logscale():
+    fig, ax = plt.subplots()
+    ax.set_xscale('log')
+    ax.set_yscale('log')
+    ax.scatter([1, 10], [1, 100])
+    rep = fig_to_dict(fig)
+    coll = rep['axes'][0]['collections'][0]
+
+    assert_equal(coll['offsetcoordinates'], 'data')
+
+
 def test_patch():
     fig, ax = plt.subplots()
     ax.add_patch(plt.Rectangle((0, 0), 1, 2, alpha=0.2, linewidth=2,

[lucasb-eyer]

I think this might close multiple issues in mpld3 proper, but I am not going to test each of them, of course:

mpld3/mpld3#488
mpld3/mpld3#227
mpld3/mpld3#487

[vladh]

Thanks for this! This looks good at first sight, but I need to do some more in-depth review, because (1) AI, (2) I want to make sure I actually understand the change well, and (3) the code currently is named to coincide with the original matplotlib function and argument names, which isn't necessary. Will get back to you. 😊

[lucasb-eyer]

1. Heh, I guessed so which is why I even mentioned it, but just FYI: I did carefully review it, but AI has the whole matplotlib internals in its head and I dont. From old issues, it looks like even Jake had no idea on why the issue exists :)
2. That's a change I did on top of AI intentionally - you can now easily diff it with the matplotlib funtion to see the actual change (which I did to review it), and if matplotlib ever changes that function, we can easily adopt the changes.

PS: I will have a couple more such AI-assisted fixes coming in the near future as I go through missing features for my use-case. All of them are open issues, and I'll always disclose when I use AI.

[aflaxman]

I think this is so cool! AI-assisted coding could be a really good match for this project. I did so (AI-assisted) review of this PR and I can confirm that it works. You can find my notes here.

[vladh]

Just checking in to say:

• Hey @aflaxman, great to see you here! Feel free to review and merge things yourself if you'd like.
• Feel free to send a PR to add contributions from your mpld3 notes to CONTRIBUTING.md and potentially RELEASING.md. In particular, adding uv notes to these documents would be great.
• I'm trying to make time to review PRs in the next week or so