WiP: Switch to using display() to show the plots

This should ensure better compatibility with the Julia package
ecosystem. Now, if Gnuplot.jl is used in an environment capable of
showing multimedia content (IJulia, VS Code, Pluto), this will take
precedence over using gnuplot's built-in viewer. In the REPL, gnuplot
viewer is used by default.

In VS Code, for example, when the "Use Plot Pane" settings is enabled,
the plots show in that pane, but when it is disabled, gnuplot viewer
is automatically used.

For people who prefer to always use the gnuplot viewer, they can
achieve that by running:

    popdisplay(Gnuplot.GnuplotDisplay())
    pushdisplay(Gnuplot.GnuplotDisplay())

This ensures that the gnuplot display has the highest priority.
However, note that VSCode ensures that its VSCodeDisplay is always the
last and IJulia also ignores this.

On the other hand, to disable the gnuplot viewer, one can run:

    Gnuplot.option.gpviewer = false

What was tested:
- [x] Plotting from REPL
- [x] VS Code
  - [x] With Plot Pane enabled
  - [x] With Plot Pane disabled
- [x] IJulia
- [x] Gnuplot.jl test suite
- [ ] DrySessions
- [x] Pluto
- [ ] Juno
- [ ] Weave.jl

Author: wentasah

src/Gnuplot.jl

@@ -222,7 +222,7 @@ Base.@kwdef mutable struct Options
         MIME"image/svg+xml"   => "svg enhanced mouse standalone dynamic background rgb 'white'",
         MIME"text/html"       => "svg enhanced mouse standalone dynamic",  # canvas mousing
         MIME"text/plain"      => "dumb enhanced ansi")
-    gpviewer::Bool = false
+    gpviewer::Bool = true
     init::Vector{String} = Vector{String}()
     verbose::Bool = false
     preferred_format::Symbol = :auto
@@ -236,14 +236,15 @@ const sessions = OrderedDict{Symbol, Session}()
 const options = Options()
 
 function __init__()
-    # Check whether we are running in an IJulia, Juno, VSCode or Pluto session.
-    # (copied from Gaston.jl).
-    options.gpviewer = !(
-        ((isdefined(Main, :IJulia)  &&  Main.IJulia.inited)  ||
-         (isdefined(Main, :Juno)    &&  Main.Juno.isactive()) ||
-         (isdefined(Main, :VSCodeServer)) ||
-         (isdefined(Main, :PlutoRunner)) )
-    )
+    # Copied from Plots - insert GnuplotDisplay before text displays, but after graphic displays such as IJulia, VSCodeDisplay, etc.
+    insert!(Base.Multimedia.displays, findlast(x -> x isa Base.TextDisplay || x isa REPL.REPLDisplay, Base.Multimedia.displays) + 1, GnuplotDisplay())
+    atreplinit(i -> begin
+        while GnuplotDisplay() in Base.Multimedia.displays
+            popdisplay(GnuplotDisplay())
+        end
+        insert!(Base.Multimedia.displays, findlast(x -> x isa REPL.REPLDisplay, Base.Multimedia.displays) + 1, GnuplotDisplay())
+    end)
+
     if isdefined(Main, :VSCodeServer)
         # VS Code shows "dynamic" plots with fixed and small size :-(
         options.mime[MIME"image/svg+xml"] = replace(options.mime[MIME"image/svg+xml"], "dynamic" => "")
@@ -583,8 +584,8 @@ function gp_write_table(args...; kw...)
     sid = Symbol("j", Base.Libc.getpid())
     gp = getsession(sid)
     reset(gp)
-    gpexec(sid, "set term unknown")
     driver(sid, "set table '$tmpfile'", args...; kw...)
+    execall(gp, term="unknown")
     gpexec(sid, "unset table")
     quit(sid)
     out = readlines(tmpfile)
@@ -767,7 +768,9 @@ function reset(gp::Session)
     gpexec(gp, "reset session")
 
     if options.gpviewer
-        # Use gnuplot viewer
+        # Use gnuplot viewer. It would be more appropriate to do this in display(), but
+        # doing so leads to hang of the gnuplot process (it receives the END marker, but
+        # does not print it back).
         (options.term != "")  &&  gpexec(gp, "set term " * options.term)
 
         # Set window title (if not already set)
@@ -784,7 +787,7 @@ function reset(gp::Session)
     end
 
     # Note: the reason to keep Options.term and .init separate are:
-    # - .term can be overriden by "unknown" (if options.gpviewer is false);
+    # - .term can be overriden by "unknown" (if options.gpviewer is false); TODO: Is this still true?
     # - .init is dumped in scripts, while .term is not;
     add_cmd.(Ref(gp), options.init)
     return nothing
@@ -1332,9 +1335,6 @@ function driver(_args...; is3d=false)
         end
     end
 
-    if options.gpviewer  &&  doDump
-        execall(gp)
-    end
     return SessionID(gp.sid, doDump)
 end
 
@@ -1547,10 +1547,23 @@ end
 
 
 # ╭───────────────────────────────────────────────────────────────────╮
-# │                     Interfacing Julia's show                      │
+# │              Interfacing Julia's Multimedia I/O                   │
 # ╰───────────────────────────────────────────────────────────────────╯
 # --------------------------------------------------------------------
 
+struct GnuplotDisplay <: AbstractDisplay end
+
+function Base.display(d::GnuplotDisplay, obj::SessionID)
+    # Use this display only when options.gpviewer is set
+    if options.gpviewer && obj.dump
+        # TODO: Shall we ignore DrySession here?
+        gp = getsession(obj.sid)
+        execall(gp)
+    else
+        throw(MethodError(display, (d, obj)))
+    end
+end
+
 function show(obj::SessionID)
     gp = getsession(obj.sid)
     @info "Gnuplot session" sid=gp.sid datasets=length(gp.datas) plots=length(gp.plots)
@@ -1560,7 +1573,7 @@ show(io::IO, gp::SessionID) = nothing
 
 
 function showable(mime::Type{T}, gp::SessionID) where T <: MIME
-    if gp.dump  &&  !options.gpviewer
+    if gp.dump
         if mime in keys(options.mime)
             term = strip(options.mime[mime])
             if term != ""

test/runtests.jl

@@ -110,6 +110,24 @@ dummy = terminals()
 # Force unknown on Travis CI
 Gnuplot.options.term = "unknown"
 
+# Wrap @gp macrocalls in display() to ensure that the whole plot is
+# sent to the gnuplot process during testing.
+macro gp(args...)
+    gpcall = :(Gnuplot.@gp)
+    push!(gpcall.args, args...)
+    out = :(display($gpcall))
+    return esc(out)
+end
+
+# Wrap @gsp macrocalls in display() to ensure that the whole plot is
+# sent to the gnuplot process during testing.
+macro gsp(args...)
+    gpcall = :(Gnuplot.@gsp)
+    push!(gpcall.args, args...)
+    out = :(display($gpcall))
+    return esc(out)
+end
+
 @gp 1:9
 @info "using terminal: " terminal()
 #test_terminal("unknown")