Describe the bug
I was working on a little app with a stacked bar chart, and came across this possible bug (black lines were added in Paint for illustrative purposes):

Each bar chart is based upon a Vec<(f64, f64)>, which contains pairs of (value, frequency), all vectors have the same length, value isn't being used as the Bar's argument intentionally.

// declared elsewhere
frequencies: Vec<Vec<(f64, f64)>>,

// plot rendering
let mut charts = vec![];
for (frequencies, label) in self.frequencies.iter().zip(self.labels.iter()) {
    let bars = frequencies
        .iter()
        .enumerate()
        .filter(|(_, (_, f))| f > &0.0)
        .map(|(i, (v, f))| Bar::new(i as f64, *f).width(self.width).name(v))
        .collect();

    let mut chart = BarChart::new(bars).name(&label);
    if !self.vertical {
        chart = chart.horizontal();
    }

    if charts.len() > 0 {
        let other_charts: Vec<_> = charts.iter().collect();
        chart = chart.stack_on(&other_charts);
    }

    charts.push(chart);
}

Zero frequencies are filtered out, so the total number of bars will vary. The reason for filtering is that a Bar with zero height will be rendered anyway, and to me it looks a little confusing (the plot's aspect ratio is different, but all values are the same):

Here, the offset values are correct, since the number of bars in both charts is the same.

To Reproduce
The easiest way to reproduce this behavior is to modify the demo app by commenting out this line:

Expected behavior
I expect the modified demo app to look like this:

Additional context
I assume it's a bug, since I never used stacked bar charts before.