Needs["PlotLegends`"]

l=10;

F= LegendreP[l,x];

dF =D[LegendreP[l,x],x];

dFn = Integrate[dF*dF,{x,-1,1}];

Plot[{F,dF, 1/Sqrt[dFn]*dF},{x,-1,1},PlotLegend->{"P_l", "dP_l", "dP_l (norm)"}]