Function element_to_png_with_background 

pub fn element_to_png_with_background(
    element: &ChartElement,
    width: u32,
    height: u32,
    density: u32,
    background: [u8; 3],
) -> Result<Vec<u8>, RenderError>

Render a pre-built ChartElement tree to PNG bytes with an explicit canvas background color. See render_to_png_with_background.