The normal "Rpad" way to make a graph:

plot(sin)
HTMLon()
showgraph()

Or, you can make a link to a graph:

plot(sin)
HTMLon()
H("span", "Click ",
  H("a", href = RpadPlotName(), "text"), " to see the plot, or ",
  H("a", href = RpadPlotName(), target="_blank", "text"), " to see the plot in a new window.")

Finally, you can make popup, but you need to use some javascript:

plot(sin)
HTMLon()
H("script", "window.open('", RpadPlotName(), "')")