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();

[somiaj]

Not related to this pull request, but TikZ doesn't allow \dfrac{}{}? I decided to see what labels would do if they were really tall and the ymin was zero, so I replaced your problem with ymin => 0 and xtick_labels => { 2 => '\(\dfrac{\dfrac{a}{b}}{\dfrac{c}{d}}\)', 6 => '\(b\)' }. The TiKz version only showed abcd for the label. But if I replaced dfrac with frac it worked. Does some additional TiKz package need loaded?

TiKz:

JSXGraph

[drgrice1]

I will look into the \dfrac thing. I am not sure what is happening there. latex is giving an undefined control sequence for a \dfrac.

[drgrice1]

Ahh, it is because \dfrac is part of the amsmath package. It is not default latex. So you will need to add the amsmath package to use it.

[drgrice1]

This is a limitation of the plots.pl macro. It does not give you access to the LaTeXImage object before it is drawn, and so there is not way to add TeX packages.

[somiaj]

Might be worth adding the amsmath package by default? I didn't consider that someone would need access to it, but seems like there is a reason. Ether way, with this people may want to use \dfrac for fraction labels, so we could address this here or another PR if we want to make a more extensible method to allow access to the LaTeXImage object.