Function hit_test_volume 

pub fn hit_test_volume(
    click_pos: Pos2,
    bars: &[Bar],
    visible_range: Range<usize>,
    coords: &ChartMapping,
    max_volume: f64,
) -> Option<SeriesHitResult>

Hit test volume bars to find which bar was clicked.

Uses the same coordinate system as the chart renderer (right-to-left).