Accept a full sampling range specification in plot with out the "samp…

…le" keyword

Background:
The plot command accepts axis ranges as the first thing after "plot".
If present, they update the primary axis ranges x y x2 y2.
Since a sampling range of the form [t=min:max] looks just like an axis
range with a renamed axis variable name this introduced a window for
ambiguity.  To remove the ambiguity, version 5 had a keyword "sample"
that interposed between axis ranges (if any) and the first sampling range.

This patch:
If a second colon is found in the range, as in [t=min:max:increment], then
it is unambiguously a sampling range rather than a primary axis range.
In this case there is no ambiguity and the plot proceeds as if there had
been a preceeding "sample" keyword.  To make this more general, a empty
field following that second colon defaults to whatever would have happened
without the colon.  Usually this is increment=(max-min)/samples or
increment=1 depending on context.  So it is always possible to avoid using
the "sample" keyword by consistently including a full sampling range
including the second color.

v5:
  plot [x=min:max][y=min:max] sample [t=min:man:increment]   # OK
  plot [x=min:max][y=min:max][t=min:man:increment]   # not accepted
  plot [x=min:max][y=min:max][t=min:man]             # misinterpreted as x2 range
v6 after this patch:
  plot [x=min:max][y=min:max] sample [t=min:man:increment]   # OK
  plot [x=min:max][y=min:max][t=min:man:increment]   # OK, increment makes it clear
  plot [x=min:max][y=min:max][t=min:man]             # still misinterpreted as x2 range
  plot [x=min:max][y=min:max][t=min:man:]            # empty increment OK, uses default

Note:
  The recognition of full sampling ranges [min:max:incr] also applies to
  splot commands, but if you mix axis ranges and sampling ranges it can still
  get confused.  To be safe, always use a full sampling range with a second colon.

Bug #2666

docs/gnuplot.doc

@@ -7871,7 +7871,7 @@ Ffigure_zerror
  `splot` draws 2D projections of 3D surfaces and data.
 
  Syntax:
-       plot {<ranges>} <plot-element> {, <plot-element>, <plot-element>}
+       plot {<axis-ranges>} <plot-element> {, <plot-element>, <plot-element>}
 
  Each plot element consists of a definition, a function, or a data source
  together with optional properties or modifiers:
@@ -9118,16 +9118,20 @@ Ffigure_smooth_path
 
  An independent sampling range can be provided immediately before the '+'. As
  in normal function plots, a name can be assigned to the independent variable.
- If given for the first plot element, the sampling range specifier has to be
- preceded by the `sample` keyword (see also `plot sampling`).
-
-       plot sample [beta=0:2*pi] '+' using (sin(beta)):(cos(beta)) with lines
-
  Here is an example where the sampling interval (1.5) is given as part of the
  sampling range.  Samples will be generated at -3, -1.5, 0, 1.5, ..., 24.
 
        plot $MYDATA, [t=-3:25:1.5] '+' using (t):(f(t))
 
+ A sampling range that immediately follows the `plot` or `splot` command might
+ be mistakenly parsed as an x-axis range.  To prevent this ambiguity you can
+ either precede it with the `sample` keyword or provide a sampling interval,
+ possibly empty to use the default, as a third field of the sampling range.
+ See the example of such ambiguity in `plot sampling`.
+
+       plot sample [beta=0:2*pi] '+' using (sin(beta)):(cos(beta)) with lines
+       plot        [beta=0:2*pi:] '+' using (sin(beta)):(cos(beta)) with lines
+
  The pseudo-file '++' returns 2 columns of data forming a regular grid of [u,v]
  coordinates with the number of points along u controlled by `set samples` and
  the number of points along v controlled by `set isosamples`.  You must set
@@ -9591,14 +9595,22 @@ Ffigure_smooth_path
        set xrange [0:50]
        plot [0:10] f(x), [10:20] g(x), [20:30] h(x)
 
- To remove the ambiguity in the previous example, insert the keyword `sample`
- to indicate that [0:10] is a sampling range applied to a single plot component
- rather than a global x-axis range that would apply to the entire plot.
-       plot sample [0:10] f(x), [10:20] g(x), [20:30] h(x)
+ To remove the ambiguity in the previous example, either insert the keyword
+ `sample` to indicate that [0:10] is a sampling range rather than an axis range
+ or add a sampling increment field (following a second colon) in the range
+ specifier.  This works because a full range specifier [min:max:increment]
+ cannot be mis-parsed as an axis range.  If the increment field is empty
+ then the increment defaults to (min-max/samples), so all three variants
+ below produce the same result.
+       set samples 100
+       plot sample [0:10]     f(x), [10:20] g(x), [20:30] h(x)
+       plot        [0:10:0.1] f(x), [10:20] g(x), [20:30] h(x)
+       plot        [0:10:]    f(x), [10:20] g(x), [20:30] h(x)
 
  This example shows one way of tracing out a helix in a 3D plot
        set xrange [-2:2]; set yrange [-2:2]
-       splot sample [h=1:10] '+' using (cos(h)):(sin(h)):(h)
+       set angle degrees
+       splot [phi=1:720:2] '+' using (cos(phi)):(sin(phi)):(phi)
 
 4 2D sampling (u and v axes)
 ?sampling 2D
@@ -9624,7 +9636,7 @@ Ffigure_smooth_path
 
  The range specifiers for sampling on u and v can include an explicit sampling
  interval to control the number and spacing of samples:
-      splot sample [u=30:70:1][v=0:50:5] '++' using 1:2:(func($1,$2))
+      splot [u=30:70:1][v=0:50:5] '++' using 1:2:(func($1,$2))
 
 3 for loops in plot command
 ?commands plot for

src/axis.c

@@ -2368,15 +2368,18 @@ char *c, *cfmt;
 /* Accepts a range of the form [MIN:MAX] or [var=MIN:MAX]
  * Loads new limiting values into axis->min axis->max
  * Returns
+ *	>0 = token indexing the leading variable name [foo=min:max]
  *	 0 = no range spec present
- *	-1 = range spec with no attached variable name
- *	>0 = token indexing the attached variable name
+ *	-1 = range spec with no leading variable name
+ *	-2 = found a second colon, so it must be a sampling range
  */
 int
 parse_range(AXIS_INDEX axis)
 {
     struct axis *this_axis = &axis_array[axis];
     int dummy_token = -1;
+    int starting_token = c_token;
+    struct axis saved_axis;
 
     /* No range present */
     if (!equals(c_token, "["))
@@ -2395,9 +2398,23 @@ parse_range(AXIS_INDEX axis)
 	c_token += 2;
     }
 
+    /* Save entire axis state before loading range, so we can back out if necessary */
+    memcpy(&saved_axis, this_axis, sizeof(struct axis));
+
     this_axis->autoscale = load_range(this_axis, &this_axis->min, &this_axis->max,
 					this_axis->autoscale);
 
+    /* A second colon is permitted only in a sampling range.
+     * If we see one now, back out from trying to interpret this as a primary axis.
+     */
+    if (equals(c_token, ":")
+    && !(axis == SAMPLE_AXIS || axis == T_AXIS || axis == U_AXIS || axis == V_AXIS)) {
+	c_token = starting_token;
+	/* restore original axis range */
+	memcpy(this_axis, &saved_axis, sizeof(struct axis));
+	return -2;
+    }
+
     /* Nonlinear axis - find the linear range equivalent */
     if (this_axis->linked_to_primary) {
 	AXIS *primary = this_axis->linked_to_primary;

src/plot2d.c

@@ -268,18 +268,25 @@ plotrequest()
 	    parse_skip_range();
     }
 
-    /* Range limits for the entire plot are optional but must be given	*/
-    /* in a fixed order. The keyword 'sample' terminates range parsing.	*/
-    if (parametric || polar) {
+    /* Axis range limits for the entire plot are optional but must be given
+     * in the fixed order x y x2 y2.
+     * A sampling range [var=start:end:increment] terminates parsing of axis
+     * range limits.  The keyword 'sample' also terminates range parsing.
+     */
+    if (parametric || polar)
 	dummy_token = parse_range(T_AXIS);
-	parse_range(FIRST_X_AXIS);
-    } else {
+    else
 	dummy_token = parse_range(FIRST_X_AXIS);
-    }
-    parse_range(FIRST_Y_AXIS);
-    parse_range(SECOND_X_AXIS);
-    parse_range(SECOND_Y_AXIS);
-    if (equals(c_token,"sample") && equals(c_token+1,"["))
+#define SAMPLING_RANGE -2
+    if (dummy_token == SAMPLING_RANGE)
+	dummy_token = 0;
+    else if ((parse_range(FIRST_Y_AXIS) != SAMPLING_RANGE)
+         &&  (parse_range(SECOND_X_AXIS) != SAMPLING_RANGE)
+         &&  (parse_range(SECOND_Y_AXIS) != SAMPLING_RANGE))
+		; /* Nothing to do */
+#undef SAMPLING_RANGE
+    /* FIXME: would like to deprecate the "sample" keyword altogether */
+    if (equals(c_token,"sample"))
 	c_token++;
 
     /* Clear out any tick labels read from data files in previous plot */
@@ -3621,7 +3628,7 @@ eval_plots()
 		if (!parametric && !polar)
 		    init_sample_range(axis_array + x_axis, FUNC);
 		sample_range_token = parse_range(SAMPLE_AXIS);
-		if (equals(sample_range_token, "u"))
+		if (sample_range_token > 0 && equals(sample_range_token, "u"))
 		    parse_range(V_AXIS);
 		dummy_func = &(this_plot->plot_function);