Thanks you so much for responding.
Setting the PlotArea's margin would be great, but I could not find that property in the radHTMLChart control. Am I missing something?
For a fix, I wound up changing the PlotArea.YAxis.LabelsApperarance.Step value from 2 (which is the value we use for our charts) back to 1. This forced a label to be created at the end of the Axis, which lengthened it enough for the series label to display.