Figure.plot: Reorder input parameters to "data, x, y"

[michaelgrund]

Description of proposed changes

Address #1479

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[seisman]

Looks good to me.

I removed the "maintenance" and "skip-changelog" labels and added the "deprecation" label because the change goes into the deprecation cycle.

[seisman]

@michaelgrund Still need to fix the test_plot_datetime test because it passes x, y, not x=x, y=y.

[michaelgrund]

@michaelgrund Still need to fix the test_plot_datetime test because it passes x, y, not x=x, y=y.

Also adjusted some of the gallery examples in which x, y is passed instead of x=x, y=y.