Function render_to_png 

pub fn render_to_png(
    chartml: &ChartML,
    yaml: &str,
    width: u32,
    height: u32,
    density: u32,
) -> Result<Vec<u8>, RenderError>

Render a ChartML YAML spec to PNG bytes (synchronous).

Runs the full pipeline: parse YAML → render ChartElement → SVG → PNG. Use this for specs with inline data and no async transforms (sql/forecast).

Arguments

• chartml — configured ChartML instance with renderers registered
• yaml — ChartML YAML specification string
• width — chart width in CSS pixels
• height — chart height in CSS pixels
• density — DPI (72 = 1x, 144 = 2x for PDF)