Implement pixel series for gr

src/backends/gr.jl

@@ -1681,6 +1681,10 @@ function gr_display(sp::Subplot{GRBackend}, w, h, viewport_canvas)
                 gr_draw_markers(series, x, y, clims)
             end
 
+        elseif st == :pixel
+            GR.setmarkertype(GR.MARKERTYPE_DOT)
+            GR.polymarker(x, y)
+
         elseif st == :contour
             GR.setspace(clims[1], clims[2], 0, 90)
             GR.setlinetype(gr_linetype(get_linestyle(series)))