feature request: options to display only positive or negative x and y axes

[virginiaMae]

per office hours discussion, proposed change would be
<graph displayXAxis="positiveOnly">
to the existing displayXAxis and displayYAxis attributes, add the following specifications:
"full" / "true" (default)
"none" / "false"
"positiveOnly"
"negativeOnly"

[dqnykamp]

To create a positive-only axis, add the attribute { straightFirst: false} to the options of the axis create function. For a negative-only axis, we'd also need to change swap the second defining point to go in the opposite direction. More work will be needed to make sure it switches dynamically when the attribute changes.

For the attribute itself, follow the logic of the simplify attribute of <math> so that boolean attribute values switch between no axis and a full axis.