what is wrong in my program with PlotLegend option (Mathematica8)?

Question

u = Sin[x];  
v = Cos[x];
w = Cos[x]^2;
y = Sin[x]^3;

Needs["PlotLegends`"]

Plot[{u, v, w, y}, {x, -10, 10}, PlotRange -> All, 
 PlotStyle -> {AbsoluteDashing[{}], Dashed, DotDashed, Dotted}, 
 PlotLegend[{{Graphics[{AbsoluteDashing[{}], 
      Line[{{7, 0.1}, {8, 0.1}}]}], 
    "a=0.02"}, {Graphics[{Dashed, Line[{{7, 0.3}, {8, 0.3}}]}], 
    "b=0.03"}, {Graphics[{DotDashed, Line[{{7, 0.5}, {8, 0.5}}]}], 
    "c=0.04"}, {Graphics[{Dotted, Line[{{7, 0.7}, {8, 0.7}}]}], 
    "d=0.07"}}]]

the error is:

An option must be a rule or a list of rules....

Coordinate Skeleton[2] should be a pair of numbers, or a Scaled or Offset form

Would You please help me!

Answer 1

The message "An option must be a rule or a list of rules..." tells you that PlotLegend should be a rule, not a function call. In other words, you should write something like

Plot[{u, v, w, y}, {x, -10, 10}, PlotRange -> All, PlotStyle -> {AbsoluteDashing[{}],
Dashed, DotDashed, Dotted}, PlotLegend->{{Graphics[{AbsoluteDashing[{}], 
Line[{{7, 0.1}, {8, 0.1}}]}], "a=0.02"}, {Graphics[{Dashed, Line[{{7, 0.3}, {8, 0.3}}]}],
"b=0.03"}, {Graphics[{DotDashed, Line[{{7, 0.5}, {8, 0.5}}]}], 
"c=0.04"}, {Graphics[{Dotted, Line[{{7, 0.7}, {8, 0.7}}]}], "d=0.07"}}]

This doesn't exactly work, but I don't know exactly what you're trying to accomplish with the legend. See if this works for you:

Plot[{u, v, w, y}, {x, -10, 10}, PlotRange -> All,
PlotStyle -> {AbsoluteDashing[{}], Dashed, DotDashed, Dotted},
PlotLegend -> {"a=0.02", "b=0.03", "c=0.04", "d=0.07"}]

You could also use the autoLegend function described in the question referenced by Sjord above. Here's an example (you'll need to load the functions described in the question prior to running this example):

g = Plot[{u, v, w, y}, {x, -10, 10}, PlotRange -> All, 
PlotStyle -> {AbsoluteDashing[{}], Dashed, DotDashed, Dotted}]

autoLegend[g, {"a=0.02", "b=0.03", "c=0.04", "d=0.07"}, 
Background -> Directive[LightOrange, Opacity[1]], 
Alignment -> {Right, Top}]