How to put a title to PlotLegends?

You are probably looking for PointLegend, but you should provide more details so others can reproduce your results.

mu = {0, 3.3, 6.7, 10, 13, 17, 20};
pl = PointLegend[mu, LegendLabel -> "mu", LegendFunction -> "Frame", 
   LegendLayout -> "Row", LegendMarkers -> Automatic];
fns = Table[n^(1/p), {p, 7}, {n, 10}];
ListLinePlot[fns, PlotMarkers -> Automatic, 
 PlotLegends -> Placed[pl, Top]]

enter image description here


You can wrap Placed around the legend. Here's an example modified from the SwatchLegend documentation:

Plot[{Sin[x], Cos[x]}, {x, 0, 5}, 
   PlotLegends -> Placed[SwatchLegend[{"first", "second"}, 
LegendLabel -> "legend title"], {0.2, 0.3}]]

enter image description here


You can use PlotLegends - > Placed[labels, Top, Labeled[#, legendlabel, Top] &] .

Using the example input from Tim Laska's answer:

mu = {0, 3.3, 6.7, 10, 13, 17, 20};
fns = Table[n^(1/p), {p, 7}, {n, 10}];

ListLinePlot[fns, PlotMarkers -> Automatic, 
 PlotLegends -> Placed[mu, Top, Labeled[#, Style["mu", 16], Top] &]]

enter image description here

You can further embellish the appearance using the third argument of Placed:

ListLinePlot[fns, PlotMarkers -> Automatic, 
 PlotLegends -> Placed[mu, Top, Panel @ Labeled[#, Style["mu", 16], Top] &]]

enter image description here

ListLinePlot[fns, PlotMarkers -> Automatic, 
 PlotLegends ->  Placed[mu, Top, 
   Framed[#, FrameStyle -> Directive[Thin, Gray], 
       RoundingRadius -> 5, ImageMargins -> 5] & @
     Labeled[#, Style[Row[{Spacer[5], "mu"}], 24, "Section"], {{Top, Left}}] &]]

enter image description here