Make it possible to set custom tick positions and labels with the plots.pl macro.

[drgrice1]

To set custom tick positions use the tick_positions axis option. Set that to a reference to an array containing the positions on the axis that ticks are desired. For example, tick_positions => [ 2, 5, 9 ] will place ticks at positions 2, 5, and 9 on the axis. Note that when this option is used the tick_delta, tick_scale, and tick_distance options are not used. So only the given tick positions will appear in the graph.

To set custom tick labels use the tick_labels option. Note that this is not a new option, but now it accepts a new type of value. Previously this was purely boolean (0 or 1), and it only determined if tick labels would be shown or not. Now it can take a value that is a reference to a hash. The keys of the hash are tick positions, and the values are the labels to be placed at those positons. Note that formatting of the label must be done by the auther, and the tick_label_format option is ignored for any label provided in this hash. If a major tick is not listed in the hash, then the position will be used for the label and it will be formatted according to the tick_label_format option.

This is intended to replace what is done in #1374 and is a more flexible approach than what is done there. In that pull request the capability for custom tick labels only is added, and it is extremely restrictive in what it can do. Only positive tick labels can be customized, and it requires that the problem author label all major ticks (there is no fallback and a tick is labeled "undefined" if one is missing).

Here is a rather rudimentary example of using this new feature:

DOCUMENT();

loadMacros('PGstandard.pl', 'PGML.pl', 'plots.pl', 'PGcourse.pl');

$plot = Plot(
    xmin            => -2,
    xmax            => 12,
    xtick_distance  => 2,
    xminor          => 1,
    xtick_positions => [ 2, 4, 6 ],
    xtick_labels    => { 2 => '\(a\)', 6 => '\(b\)' },
    ymin            => -2,
    ymax            => 12,
    ytick_distance  => 2,
    yminor          => 1,
);

BEGIN_PGML
[@ $plot->image_type('Tikz'); @]*[!TikZ Image!]{$plot}

[@ $plot->image_type('JSXGraph'); @]*[!JSXGraph Image!]{$plot}
END_PGML

ENDDOCUMENT();