Legend Improvements 

Legends automatically scale the markers when there is dashing or thickness to make it easier to identify the curves.

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X {Plot[{Sin[x], Cos[x]}, {x, 0, 2 \[Pi]}, PlotLegends -> Placed["Expressions", Below]], Plot[{Sin[x], Cos[x]}, {x, 0, 2 \[Pi]}, PlotStyle -> {Dashed, Thickness[.02]}, PlotLegends -> Placed["Expressions", Below]]}

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Legends automatically use the same colors as surfaces.

In[2]:=

X {Plot3D[{Sin[x y], Cos[x y]}, {x, 0, Pi}, {y, 0, Pi}, PlotLegends -> Placed["Expressions", Below]], Plot3D[{Sin[x y], Cos[x y]}, {x, 0, Pi}, {y, 0, Pi}, PlotStyle -> {Red, Blue}, PlotLegends -> Placed["Expressions", Below]]}

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