What decides if the span is on the axis line, or above/below the axis line?   In some of the examples in the documentation I can see it sometimes move spans above the line, even if there is just one series.  I have 3 data series and they all end up on the axis, even if I can't set where is puts it, why do some of the examples show it?

Example from Documentation with 1 data series 

If the specific event entry has Duration equal to 0, it shows a point on the axis line. If the event entry Duration is greater than 0, it shows a span above or below the axis line depending on the position of the Series (Series.Positon property).

If you want to change the styles of the event points/spans, you can look here:

Try changing the style to:

<Style x:Key="EventSpan_1" TargetType="igtl:EventSpan">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="igtl:EventSpan">
                        <Grid Margin="0,2,0,2">
                            <Rectangle
               Height="10"
               Fill="Orange"
               RadiusX="7"
               RadiusY="7"
               Stroke="Black"

VerticalAlignment="Top"

               StrokeThickness="1"/>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>

About the default styles you can look here: