[robertoffmoura]

mpl_to_plotly drops the arrows of matplotlib quiver plots. plt.quiver creates a Quiver collection, which the exporter passes to draw_path_collection like any other path collection. The renderer turns it into a scatter of circle markers at the arrow positions with an empty marker size list, so the converted figure shows empty axes.

Fix: draw_path_collection now draws Quiver collections as plotly quiver traces (added in plotly.js 4.0.0, bundled since plotly.py 7.0.0):

• Each arrow's tail and tip are read from matplotlib's arrow path, so arrow lengths, directions and pivots ('tail', 'mid', 'tip') follow the matplotlib figure. Each arrow is anchored at its tail, with u/v reaching its tip in axis units: data units on linear axes, decades on log axes and milliseconds on date axes.
• Arrow positions given in data, axes, figure or display coordinates convert, on linear, log and date axes.
• Arrows of each color go into their own trace, so per-arrow and colormapped colors are kept. Arrows drawn only as outlines (facecolor="none") take the edge color.
• Masked and NaN arrows are skipped.

Snippet to reproduce:

import datetime
import matplotlib

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


def save_comparison(fig, name):
    fig.savefig(f"{name}_mpl.png", bbox_inches="tight")
    p = tls.mpl_to_plotly(fig)
    p.write_image(f"{name}_plotly.png")


# 1. Vector field
x = np.arange(-3, 3.5, 0.5)
X, Y = np.meshgrid(x, x)
fig1, ax1 = plt.subplots()
ax1.quiver(X, Y, np.sin(X), np.cos(Y))
save_comparison(fig1, "quiver_field")

# 2. Colormapped arrows
x = np.linspace(-2, 2, 8)
X, Y = np.meshgrid(x, x)
fig2, ax2 = plt.subplots()
ax2.quiver(X, Y, -Y, X, Y + X / 8)
save_comparison(fig2, "quiver_colormapped")

matplotlib	plotly before	plotly after