I figured it out, and just made a simple line - scatter plot.
The trick is, start with the Line chart, multiple series, then edit one or more of the series to Marker using the Series Type select list.
When you start with a Scatter Plot, though this may be closer to what you want in the end, the Series Type select list is not there.
Reason: The query format for a scatter plot is different - one cannot dynamically change the series type unless the same query format applies.
I found a better way for this specific case.
I really want a scatter plot, because I want all points plotted and it is nice to have the legend generated automatically.
Instead of plotting the "frame" lines as line series,
I used a background image for the static "frame: lines, added the background image with the <data_plot_background> tag,
and I added set bounds for the X and Y axis, ensure the plot area is the same each time - which is what I want for this chart:
<fill enabled="True" type="Image" image_url="./CustomCharts/Frame_Background.png" image_mode="Stretch" />
The combination of image_mode = "Stretch" and a set image size ensures my frame lines will line up as they should.
Eliminates a lot of custom code just to generate a scatter-line plot.