gnuplot bad-dat parsing bug?

Hi. I'm observing a data-parsing problem that I think is a bug... but
looking at the code, maybe it isn't? This has been an issue for a long
time; it's not something new in 6.0.

To reproduce, I have this script:

  set terminal dumb 80,40
  unset grid
  plot '-' with linespoints
  1  -
  2  10
  3  5
  4  13
  5  4
  6  13
  7  7
  8  15
  e

Note the bad-data line "1 -". This produces this plot:


  8 +-----------------------------------------------------------------------+
    |         +          +         +         +         +          +      ** |
    |                                                           '-' ***A*** |
    |                                                                **     |
    |                                                              **       |
  7 |-+                                                          *A       +-|
    |                                                          **           |
    |                                                        **             |
    |                                                      **               |
    |                                                    **                 |
  6 |-+                                               *A*                 +-|
    |                                               **                      |
    |                                             **                        |
    |                                           **                          |
    |                                         **                            |
  5 |-+                                      A                            +-|
    |                                      **                               |
    |                                    **                                 |
    |                                   *                                   |
    |                                 **                                    |
    |                               **                                      |
  4 |-+                           *A                                      +-|
    |                           **                                          |
    |                         **                                            |
    |                       **                                              |
    |                     **                                                |
  3 |-+                 *A                                                +-|
    |                 **                                                    |
    |               **                                                      |
    |             **                                                        |
    |           **                                                          |
  2 |-+      *A*                                                          +-|
    |      **                                                               |
    |    **                                                                 |
    |  **                                                                   |
    |**       +          +         +         +         +          +         |
  1 +-----------------------------------------------------------------------+
    0         1          2         3         4         5          6         7

Note that I would expect an up/down zigzag, which ignores the whole
bad-data line. Removing that line produces the expected output:


  16 +----------------------------------------------------------------------+
     |           +           +           +          +           +           |
     |                                                          '-' ***A*** |
     |                                                                      |
     |                                                                      |
     |                                                                     *|
  14 |-+                                                                  +*|
     |                                                                    * |
     |                                                                    * |
     |                       A                      A                    *  |
     |                       *                      **                   *  |
     |                      * *                    * *                  *   |
  12 |-+                    * *                    *  *                 * +-|
     |                     *   *                  *    *               *    |
     |                     *   *                  *    *               *    |
     |                    *     *                 *     *             *     |
     |                    *     *                *       *            *     |
     |                   *       *               *       *           *      |
  10 |-+                 *       *              *         *          *    +-|
     |*                 *        *              *          *        *       |
     | *                *         *             *          *        *       |
     | *               *          *            *            *      *        |
     |  *              *           *           *             *     *        |
     |   *            *            *          *              *    *         |
   8 |-+  *           *             *         *               *   *       +-|
     |     *         *              *        *                 * *          |
     |     *         *               *       *                 * *          |
     |      *       *                *       *                  A           |
     |       *      *                *      *                               |
     |        *    *                  *     *                               |
   6 |-+       *   *                  *    *                              +-|
     |         *  *                    *   *                                |
     |          * *                    *   *                                |
     |           A                      * *                                 |
     |                                  * *                                 |
     |           +           +           *          +           +           |
   4 +----------------------------------------------------------------------+
     2           3           4           5          6           7           8

I can also "fix" it by adding to the script:

  set datafile missing "-"

What should the default behavior be? Should it not be ignoring the
bad-data line entirely? The block of code that handles this is here:

  https://github.com/gnuplot/gnuplot/blob/fbeb88eadedf927a4d778b41dd118e373f33eacb/src/datafile.c#L2380

With this data we have df_column[1].good == DF_BAD. The code checks for
DF_MISSING and DF_UNDEFINED, but not DF_BAD. We end up exiting the loop here:

  https://github.com/gnuplot/gnuplot/blob/fbeb88eadedf927a4d778b41dd118e373f33eacb/src/datafile.c#L2425

which leaves output==1. And then we set df_no_use_specs = output = 1 a
bit later:

  https://github.com/gnuplot/gnuplot/blob/fbeb88eadedf927a4d778b41dd118e373f33eacb/src/datafile.c#L2458

So the first row of data has one valid column, and we then expect
exactly one valid column from every subsequent row of data as well.
Checked here:

  https://github.com/gnuplot/gnuplot/blob/fbeb88eadedf927a4d778b41dd118e373f33eacb/src/datafile.c#L742

Is this what we want?

Thanks much


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