Convert axhline, axvline, and axline to layout shapes

[robertoffmoura]

mpl_to_plotly silently drops reference lines. axhline, axvline, and axline produce lines with blended or axes transforms, which the renderer skipped with "I found a line that didn't have 'data' coordinates! I will not draw it, but I did not crash.". The converted figure ends up with the reference lines missing completely.

Fix: reference lines are now drawn as layout line shapes:

• endpoints are mapped to data coordinates using ax.transData.inverted()
• added as go.layout.Shape(type="line") on the current subplot
• color, alpha, linewidth, and dash styles are preserved

Before: 0 shapes (lines dropped with a warning).
After: lines render as layout shapes spanning the axis limits.

Snippet to reproduce:

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

cases = {
    "axhline": lambda: plt.axhline(0.5, color="red", linestyle="--"),
    "axvline": lambda: plt.axvline(2.0, color="blue", linestyle="-"),
    "axline":  lambda: plt.axline((1, 0.5), slope=0.5, color="green", linestyle="-."),
}

for name, build in cases.items():
    fig, ax = plt.subplots()
    ax.set_xlim(0, 4)
    ax.set_ylim(0, 2)
    build()

    p = tls.mpl_to_plotly(fig)  # 0 shapes before this fix
    print(f"{name:10s} -> {len(p.layout.shapes)} shape(s)")
    p.write_image(f"{name}_plotly.png")

axhline	axline	axvline