fix recipe error

src/backends/gr.jl

@@ -1139,7 +1139,7 @@ function gr_display(sp::Subplot{GRBackend}, w, h, viewport_canvas)
         end
 
         if st in (:path, :scatter, :straightline)
-            if length(x) > 1
+            if x != nothing && length(x) > 1
                 lz = series[:line_z]
                 segments = iter_segments(series)
                 # do area fill

src/backends/pyplot.jl

@@ -216,6 +216,7 @@ end
 # ---------------------------------------------------------------------------
 
 function fix_xy_lengths!(plt::Plot{PyPlotBackend}, series::Series)
+    if series[:x] != nothing
         x, y = series[:x], series[:y]
         nx, ny = length(x), length(y)
         if !isa(get(series.plotattributes, :z, nothing), Surface) && nx != ny
@@ -226,6 +227,7 @@ function fix_xy_lengths!(plt::Plot{PyPlotBackend}, series::Series)
             end
         end
     end
+end
 
 py_linecolor(series::Series)          = py_color(series[:linecolor])
 py_markercolor(series::Series)        = py_color(series[:markercolor])

src/utils.jl

@@ -194,7 +194,9 @@ end
 
 function iter_segments(series::Series)
     x, y, z = series[:x], series[:y], series[:z]
-    if has_attribute_segments(series)
+    if x == nothing
+        return UnitRange{Int}[]
+    elseif has_attribute_segments(series)
         if series[:seriestype] in (:scatter, :scatter3d)
             return [[i] for i in 1:length(y)]
         else