In version 5.0.23 HorizontalLine and VerticalLine in legends include a marker. I believe this is probably unintentional as there are no markers shown on the lines themselves. An example legend is shown below.

The fix is pretty trivial. In AxisLine.cs the LegendItems property definition should have Marker set to MarkerStyle.None as shown below.

    public IEnumerable<LegendItem> LegendItems
    {
        get
        {
            return LegendItem.Single(new LegendItem()
            {
                Label = Label.Text,
                Line = LineStyle,
                Marker = MarkerStyle.None,
            });
        }
    }

I'm happy to make a PR to fix this but I'd like to wait on the outcome of #3612 to make any subsequent merge easier.