Plotting Limit Lines on Chart

Unfortunately, this is a customization.  You will need to modify the out of the box chart widget to accept the upper and lower x values for your line start/end and then draw those lines on new blank canvas that overlays the chart widget.  Or use a different chart library that offers this functionality, pass in the required data, and let the library handle the rendering.

I'll mention what I ended up actually doing in case it helps someone else:

I already had a "helper" Thing created for services I needed to write.  I wrote a simple service to get the first and last entries in my InfoTable (ordered by time stamp) and create a 2 row Info Table containing these times with the same limit Number value.  Plotting this Info Table in a Time Series Chart gives a horizontal line at the y-value limit (as long as you have the Chart type set up for a Line).  I also edited the Styles of the pens to give a dotted or dashed line.