Combine Legends from Several Plots 

In[1]:=

X legendoptions = {LabelStyle -> {Bold, GrayLevel[0.3], 14}, LegendMarkerSize -> {15, 10}};

In[2]:=

X SeedRandom[44]; data = RandomVariate[NormalDistribution[], 100];

Combine two plots and their respective legends.

In[3]:=

X normalplot = Plot[PDF[NormalDistribution[], x], {x, -3, 3}, PlotStyle -> Directive[Thick, Gray], Filling -> Axis, FillingStyle -> LightGray, PlotLegends -> Placed[LineLegend[{"normal"}, legendoptions], Above]]

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In[4]:=

X histogram = SmoothHistogram[data, PlotStyle -> Directive[Thick, Orange], PlotLegends -> Placed[LineLegend[{"data"}, legendoptions], Above]]

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In[5]:=

X Grid[{{Show[normalplot, Ticks -> None, ImageSize -> 150], Show[normalplot, histogram, PlotRange -> All, ImageSize -> 350]}, {Show[histogram, Ticks -> None, ImageSize -> 150], SpanFromAbove}}, Frame -> All, Alignment -> {Left, Center}]

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