This PR addresses the long-standing feature request for gradient/highlight-styled markers in Matplotlib's plot and scatter APIs, as raised in #8926. Currently, Matplotlib lacks native support for glossy/highlighted marker rendering across backends, with workarounds like post-processing SVG files or using custom images being the only options—both of which have limitations with legend compatibility and ease of use.

Implementation Method

This change implements basic highlight marker functionality exclusively for the SVG backend by introducing a dynamic _highlight_svg member injected into Line2D (for plot) and PathCollection (for scatter) classes. The draw methods of these two classes are modified to check for the _highlight_svg flag, and the RendererSVG.draw_markers method is updated to render a highlight mask over markers when the flag is set, achieving a glossy highlight effect.

Extensibility

The implementation is intentionally minimal and unrefined at this stage, but the core design is backend-agnostic and extensible: the same flag-based injection pattern can be adapted to other backends (e.g., Agg) with backend-specific rendering logic. For the Agg backend, this would require modifications to the underlying C language code for marker drawing, which is left as future work.

Minimum self-contained example

import matplotlib.pyplot as plt
import numpy as np


x1 = [1, 2, 3, 4]
y1 = np.ones_like(x1)
np.random.seed(19680801)
x2 = np.linspace(1, 4, 11, endpoint=True)
y2 = np.random.rand(11) + 2

fig, ax = plt.subplots(figsize=(4, 3))

(line,) = ax.plot(
    x1,
    y1,
    marker="o",
    markersize=10,
    ls="-",
    lw=2,
    label="A: line",
    alpha=1,
    mew=1,
    mec="gray",
    markerfacecolor="dimgray",
    # markerfacecoloralt="gray",
    # fillstyle="left",
)
line._highlight_svg = True

(line2,) = ax.plot(
    x1,
    y1 + 1,
    marker="D",
    markersize=10,
    lw=2,
    color="#6a9f1b",
    mec="gray",
    label="B: line",
    alpha=0.75,
)
line2._highlight_svg = True

pc = ax.scatter(
    x2,
    y2,
    s=100,
    lw=1,
    color="salmon",
    ec="gray",
    label="C: scatter",
)
pc._highlight_svg = True


ax.legend(loc=0, ncol=3, frameon=True, bbox_to_anchor=(0, 0.2, 1, 0.2))
ax.set_title("highlight marker for scatter and line plot\n(SVG Backend)")

plt.savefig("main.svg", dpi=144)
plt.savefig("main.png", dpi=144) # no effect
plt.savefig("main.svg.xml", format="svg", dpi=144)
plt.show() # no effect

Known issues

A fixed element ID - _m_overlayGradient is used in the SVG backend, and this content is written to the SVG file regardless of whether the image contains highlighted markers or not.

Closes #8926

AI Disclosure

No generative AI tools were used in the development of this PR. All code and documentation were written manually with reference to Matplotlib's existing backend and marker rendering codebase.

PR checklist

• "closes Maker with gradient filling for plot and scatter [feature request] #8926" is in the body of the PR description to link the related issue
• Plotting related features are demonstrated in an [example] above
• new and changed code has not been tested
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines