Monochrome red to black gradient palette editor

/* https://grabient.com/_fxJfxufwgfcQft-fsagCGgJdgFpgYtgkLgGK?style=linearGradient&steps=2&angle=180 */

background: linear-gradient(180deg, #ff0000 0.000%, #000000 100.000%);

<svg xmlns="http://www.w3.org/2000/svg" width="800" height="400" viewBox="0 0 800 400" preserveAspectRatio="none">
            <!-- https://grabient.com/_fxJfxufwgfcQft-fsagCGgJdgFpgYtgkLgGK?style=linearGradient&steps=2&angle=180 -->
            <defs>
              <linearGradient id="gradient" x1="0.500" y1="0.000" x2="0.500" y2="1.000">
        <stop offset="0.000" stop-color="#ff0000" /><stop offset="1.000" stop-color="#000000" />
              </linearGradient>
            </defs>
            <rect x="0" y="0" width="800" height="400" fill="url(#gradient)" />
          </svg>

["#ff0000", "#000000"]

// https://grabient.com/_fxJfxufwgfcQft-fsagCGgJdgFpgYtgkLgGK?style=linearGradient&steps=2&angle=180
// cosine gradient palette — https://iquilezles.org/articles/palettes/
const float STEPS = 2.0;
const float ANGLE = 180.0;

vec3 palette(float t) {
    vec3 a = vec3(-0.951, -0.914, -0.992);
    vec3 b = vec3(-2.288, -1.154, -1.254);
    vec3 c = vec3(0.134, 0.605, 0.361);
    vec3 d = vec3(1.581, 2.315, 0.394);
    return a + b * cos(6.283185 * (c * t + d));
}

// N stops sampled from the palette, blended like the CSS gradient
vec3 stops(float t) {
    float n = STEPS - 1.0;
    float x = clamp(t, 0.0, 1.0) * n;
    return mix(palette(floor(x) / n), palette(min(floor(x) + 1.0, n) / n), fract(x));
}

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    vec2 p = fragCoord - 0.5 * iResolution.xy;
    // CSS angle: 0 = up, clockwise; span = CSS gradient-line length
    vec2 dir = vec2(sin(radians(ANGLE)), cos(radians(ANGLE)));
    float len = abs(iResolution.x * dir.x) + abs(iResolution.y * dir.y);
    float t = dot(p, dir) / len + 0.5;
    fragColor = vec4(stops(t), 1.0);
}

[
  [-0.951, -0.914, -0.992],
  [-2.288, -1.154, -1.254],
  [0.134, 0.605, 0.361],
  [1.581, 2.315, 0.394]
]