tracing – Adding lines to `PlotLegends` with` Show`

I'm trying to create parcels and then combine them with Show, but I do not manage to associate colorful lines with PlotLegends:

demoPlot1 =
Ground[5*Sinc[Pi*x], {x, -10, 10}, GridLines -> All, PlotRange -> All,
PlotLegends -> "demoPlot1"];
demoPlot2 =
Ground[Style[5*Cos[Pi*x], Lighter[Orange]], {x, -10, 10},
GridLines -> All,
PlotRange -> All, PlotLegends -> "demoPlot2"];
halfValuePlot =
Ground[Style[Range[-100, 100] - 1/2, lighter[Purple], Dashed],
{x, -100, 100}, PlotRange -> All,
PlotLegends -> "Half Values"];
Show[demoPlot1, demoPlot2, halfValuePlot, GridLines -> {units, units}, 
 PlotRange -> {{-5, 5}, {-5, 5}}]

The resulting plot contains captions, but no colored lines (or dotted lines) are displayed next to indicate the key. How can I fix that?