plot3: Draw marker when only a single point is plotted.

Although this is not Matlab-compatible, it is very confusing to
issue a plot command and have nothing drawn.  Octave had already
made the choice for the 2-D plot command to draw a marker for
a single point so now the 2-D and 3-D cases behave the same.

* NEWS.10.md: Announce change in Graphics Backend section.

* plot3.m: Check input data for being a single point and set 'marker' to '.'
and 'linestyle' to '-'.

etc/NEWS.10.md

@@ -48,7 +48,10 @@ Summary of important user-visible changes for version 10 (yyyy-mm-dd):
   `'rtick'` by the function `rticks` will only include the center tick mark
   value if it is specified.
 
-- `view` correctly interprets cartesian viewpoints on main axes (bug #65641).
+- `view` correctly interprets Cartesian viewpoints on main axes (bug #65641).
+
+- `plot3` now draws a single marker if only one data point is given.
+  Previously the plot was blank (`marker` = "none") which was confusing.
 
 ### Matlab compatibility
 

scripts/plot/draw/plot3.m

@@ -212,7 +212,13 @@
           linestyle = options.linestyle;
           marker = options.marker;
           if (isempty (marker) && isempty (linestyle))
-             [linestyle, marker] = __next_line_style__ ();
+            if (isscalar (x))
+              ## If unspecified, marker for a point is always "."
+              linestyle = "-";
+              marker = ".";
+            else
+              [linestyle, marker] = __next_line_style__ ();
+            endif
           endif
           color = options.color;
           if (isempty (color))
@@ -268,7 +274,13 @@
           linestyle = options.linestyle;
           marker = options.marker;
           if (isempty (marker) && isempty (linestyle))
-            [linestyle, marker] = __next_line_style__ ();
+            if (isscalar (x))
+              ## If unspecified, marker for a point is always "."
+              linestyle = "-";
+              marker = ".";
+            else
+              [linestyle, marker] = __next_line_style__ ();
+            endif
           endif
           color = options.color;
           if (isempty (color))
@@ -344,7 +356,13 @@
         linestyle = options.linestyle;
         marker = options.marker;
         if (isempty (marker) && isempty (linestyle))
-          [linestyle, marker] = __next_line_style__ ();
+          if (isscalar (x))
+            ## If unspecified, marker for a point is always "."
+            linestyle = "-";
+            marker = ".";
+          else
+            [linestyle, marker] = __next_line_style__ ();
+          endif
         endif
         color = options.color;
         if (isempty (color))