How to add legend for filled and unfilled regions?

Question

I have the following plot. How I add the legend for the filled, not-filled regions and the boundary line with the names "above", "below" and "boundary line", respectively? Something similiar to the graph below. The idea looks simple but I couldn't figure out how to label for a region.

Show[
 Plot[{2 Sin[x], 3 x - 2, x^2}, {x, 0, 3}, GridLines -> Automatic],
 ListLinePlot[{{0, 1}, {1, 1}, {10, 10}}, 
  PlotStyle -> Directive[Red, Dashed, Thickness[0.008]], Filling -> Top, 
  FillingStyle -> Opacity[0.75, LightGray]]]

Answer 1

Legended[Show[
  Plot[{2 Sin[x], 3 x - 2, x^2}, {x, 0, 3}, GridLines -> Automatic], 
  ListLinePlot[{{0, 1}, {1, 1}, {10, 10}}, 
   PlotStyle -> Directive[Red, Dashed, Thickness[0.008]], 
   Filling -> Top, FillingStyle -> Opacity[0.75, LightGray]]], 
 Placed[Column@{SwatchLegend[{Opacity[.5, LightGray], White}, 
     {"Above", "Below"}, LegendMarkerSize -> 25], 
    LineLegend[{Directive[Thick, Dashed, Red]}, {"Boundary"}, 
     LegendMarkerSize -> 25]}, Right]]

Alternatively, use LineLegend with the (undocumented) option "LegendItem" to inject desired glyphs as legend icons:

linelegend = LineLegend[{Opacity[.5, LightGray], Directive[Red, Thick, Dashed], White},
  {"Above", "Boundary", "Below"}, 
  "LegendItem" -> {"Square", Automatic, "Square"}, LegendMarkerSize -> 25]

and use linelegend as the setting for PlotLegends in any of the plots in Show:

Show[Plot[{2 Sin[x], 3 x - 2, x^2}, {x, 0, 3}, GridLines -> Automatic,
   PlotLegends -> linelegend], 
 ListLinePlot[{{0, 1}, {1, 1}, {10, 10}}, 
  PlotStyle -> Directive[Red, Dashed, Thickness[0.008]], 
  Filling -> Top, FillingStyle -> Opacity[0.75, LightGray]]]

The second approach is quite convenient for using arbitrary graphics objects/images as legend icons, including several built-in objects (like "Square", "Curve", "Doodle" and "MarkerCluster") and built-in ChartElementData objects (like, for instance, "TriangleWaveBubble", "NoiseBubble", "SquareWaveBubble", etc.):

styles = Prepend[Directive[Red, Thick, Dashed]]@(ColorData[97] /@ Range[9]);

labels = {"Automatic", "Square", "Curve", "Doodle", "MarkerCluster", 
   "NoiseBubble", "SquareWaveBubble", "TriangleWaveBubble", 
   "Mandrill", "FreakedSmiley"};

legenditems = {Automatic, "Square", "Curve", "Doodle", 
   "MarkerCluster", ChartElementData["NoiseBubble"], 
   ChartElementData["SquareWaveBubble"], 
   ChartElementData["TriangleWaveBubble"], 
   ImageMultiply[ExampleData[{"TestImage", "Mandrill"}], ColorData[97]@8], 
   Graphics[{ColorData[97]@9, Text@Style["\[FreakedSmiley]", 62]}]};

LineLegend[styles, labels, "LegendItem" -> legenditems, 
 LegendMarkerSize -> 50, LegendLayout -> {"Column", 2}]