Vtyjkyuk

So I am having a similar problem to the one in this question but the solutions therein are of no use.

Consider a basic ContourPlot with an automatic BarLegend

ContourPlot[Cos[y + x] x y, x, 0, 3, y, 0, 3, PlotRange -> All, 
 PlotLegends -> BarLegend[Automatic], 
Epilog -> Red, Line[Table[i, Cos[i], i, 0., 3., .1]]]

which gives the following, nice ContourPlot:

https://imgur.com/KV5kD1m

However, when I try and add a LineLegend for the Epilog, ContourPlot cannot do so automatically. I have tried all suggestions in the link above and none of them work

ContourPlot[Cos[y + x] x y, x, 0, 3, y, 0, 3, PlotRange -> All, 
 PlotLegends -> 
 Column[BarLegend[Automatic], 
LineLegend[Directive[Red], "Test"]], 
Epilog -> Red, Line[Table[i, Cos[i], i, 0., 3., .1]]]

https://imgur.com/OP9Mzoa

How do I fix this? I want the BarLegend to be automatic, since it provides a nice, simple fit of values. However, even manually I have had no success with BarLegend not evaluating.

(As a sidenote, imgur is not accepting the pictures I have provided. Have put up links, if someone can fix, would be grateful).

PlotLegends can take a list of legends:

ContourPlot[Cos[y + x] x y, x, 0, 3, y, 0, 3, 
 PlotRange -> All, PlotLegends -> 
 BarLegend[Automatic], 
 Placed[LineLegend[Directive[Red], "Test"], After]
 , 
 Epilog -> Red, Line[Table[i, Cos[i], i, 0., 3., .1]]]

By default one legend will go to the right, and one below, so I used Placed to put both on the right.

Here is a work-around using separate ContourPlots:

p1 = ContourPlot[Cos[y + x] x y, x, 0, 3, y, 0, 3, 
 PlotRange -> All, 
 PlotLegends -> BarLegend[Automatic]];

p2 = ContourPlot[y == Cos[x], x, 0, 3, y, 0, 3, 
 ContourStyle -> Red, 
 PlotLegends -> LineLegend["Test"]];

Show[p1, p2]

Will this be ok?

Each plot type allows only specific legends to be used with PlotLegends, for ContourPlot and DensityPlot it is BarLegend. So, it is getting confused when you add LineLegend. (Per Marius, there is a switchover to LineLegend when only specific contours are used, e.g. f == g form of input to ContourPlot.)

The correct way is to use Legended to add the additional legend:

Legended[
 ContourPlot[Cos[y + x] x y, x, 0, 3, y, 0, 3, 
 PlotRange -> All, PlotLegends -> BarLegend[Automatic], 
 Epilog -> Red, Line[Table[i, Cos[i], i, 0., 3., .1]]]
 , 
 LineLegend[Directive[Red], "Test"]
]

Hacked it. The key was to use LegendLabel to label the BarLegend and then hack it into what I wanted.

ContourPlot[Cos[y + x] x y, x, 0, 3, y, 0, 3, PlotRange -> All, 
PlotLegends -> 
BarLegend[Automatic, 
 LegendLabel -> 
Placed[LineLegend[Directive[Red], "Test"], Bottom]], 
Epilog -> Red, Line[Table[i, Cos[i], i, 0., 3., .1]]]

The result is

https://imgur.com/a/2OdeovA