Add grid_color and grid_fade API

[gavrelina]

Summary

• grid_color(Color32) — sets the base color for grid lines independently of text_color(), while preserving the strength-based fading.
• grid_fade(f32) — controls the contrast between dense and sparse grid lines. 0.0 = uniform (all lines same opacity), 0.5 = default (sqrt), 1.0 = maximum fade. The zoom-based density logic is always preserved.

Motivation

When embedding egui_plot in an application with its own design system, the grid color needs to be independent of text color. The current behavior derives grid color from ui.visuals().text_color(), which couples grid styling to text styling.

Similarly, the strength curve (hardcoded sqrt) may not suit all visual designs — some applications want less contrast between grid levels, or uniform grid lines.

Changes

Single file change in plot.rs:

• Added grid_color: Option<Color32> field with .grid_color() builder method
• Added grid_strength_exponent: f32 field (default 0.5) with .grid_fade() builder method
• paint_grid_direction uses the custom base color and exponent when set, falls back to existing behavior otherwise

[github-actions]

View snapshot changes at kitdiff