[robertoffmoura]

mpl_to_plotly silently drops reference spans. axhspan and axvspan add a Rectangle patch with a blended axes/data transform, which the renderer skipped with "I found a path object that I don't think is part of a bar chart. Ignoring.". The converted figure ends up with the spans missing completely.

Fix: reference spans are now drawn as layout rectangle shapes:

• a span is recognized as an axis-aligned rectangle in a blended axes/data transform (matplotlib gives spans no other marker); other shapes in such transforms keep the warning
• sides in data coordinates map onto the data axis, and sides in axes fractions (the full width/height, or xmin/xmax, ymin/ymax) map onto the axis domain (x domain/y domain), so they stay fixed to the axes on pan and zoom
• x-values are converted to date strings on date axes, and spans work on log axes
• fill color, edge color, alpha, linewidth and dash styles are preserved
• labeled spans get a legend entry

Before: 0 shapes (spans dropped with a warning).
After: spans render as layout shapes.

Snippet to reproduce:

import datetime

import matplotlib

matplotlib.use("Agg")
import matplotlib.pyplot as plt
import plotly.tools as tls


def spans(ax):
    ax.plot([0, 10], [0, 10])
    ax.axhspan(2, 4, color="orange", alpha=0.5)
    ax.axvspan(6, 8, color="green", alpha=0.5)


def partial_spans(ax):
    ax.plot([0, 10], [0, 10], label="line")
    ax.axhspan(2, 4, xmin=0.2, xmax=0.6, color="orange", label="axhspan")
    ax.axvspan(6, 8, ymin=0.1, ymax=0.5, color="green")
    ax.legend()


def spans_date_axis(ax):
    dates = [datetime.datetime(2023, 1, day) for day in range(1, 10)]
    ax.plot(dates, range(9))
    ax.axvspan(dates[2], dates[5], color="purple", alpha=0.3)
    ax.axhspan(2, 4, facecolor="red", alpha=0.3, edgecolor="black", linestyle="--")


cases = {
    "spans": spans,
    "partial_spans": partial_spans,
    "spans_date_axis": spans_date_axis,
}

for name, build in cases.items():
    fig, ax = plt.subplots()
    build(ax)
    fig.savefig(f"{name}_mpl.png")

    p = tls.mpl_to_plotly(fig)  # 0 shapes before this fix
    print(f"{name:16s} -> {len(p.layout.shapes)} shape(s)")
    p.write_image(f"{name}_plotly.png")

matplotlib	plotly before	plotly after

[camdecoster]

Thanks for the PR! I'll review and follow up.