Fix some issues with plots.pl and the graph tool.

[drgrice1]

First, fix the alt attribute for TikZ and GD image types for plots.pl.

The image method of PGbasicmacros.pl currently sets the aria_description with the value of the alt option if it is passed to the method, and it does so regardless of the plots image type. However, that is only used for html output with JSXGraph. Thus for TikZ or GD image types the alt attribute is lost. This makes it so that the aria_description is only set for html output, and for all other outputs the given alt tag is left in the @alt_list so that the later code that inserts the <image> tag can get it and add it as an attribute to the <image> tag.

You can test this with the following MWE:

DOCUMENT();

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

$plot = Plot();
$plot->add_function('x^2', 'x', -10, 10, color => 'blue');
$plot->image_type('tikz');

BEGIN_PGML
[!graph of x^2!]{$plot}{300}
END_PGML

ENDDOCUMENT();

With that example and the current code, the image will not have an alt attribute, but will with this pull request. If you remove the line that sets the image type to 'tikz', then the JSXGraph image will get the aria description (with the second fix below).

Second, fix the aria description for both JSXGraph output of plots.pl and the graph tool.

This is caused by the removal of the description option for the JXG.Board object in the JSXGraph library. I must have missed this when this happened three years ago. Although, it seems to have been done rather quietly, as this is not listed in the change log for JSXGraph.

To fix this, I just do the same thing that the description option used to do, and add a visually hidden span that the graph is aria-describedby. Note that there is a new aria-description attribute that could be used in the future for this, but it is in a future aria specification, and I don't know how well supported it is at this point.

Finally, fix some issues with GD output of the plots.pl macro. This is caused when an Plots::Plot object does not have the height explicitly set. For TikZ and JSXGraph output, the size method is called which determines the height if it is not set explicitly. So GD output should do the same.