This ticket implements new show options flip_x and flip_y for flipping the direction of axes in 2d graphics. This was previously suggested in #18463 comment:12.

Currently, something similar can be achieved by setting limits such that xmin > xmax.

sage: plot(x).show(xmin=1, xmax=-1)

This works because it is supported by matplotlib, but it is fragile and does not seem to be documented anywhere in Sage, apart from some implementation comments in sage.plot.graphics. The implementation of matrix_plot also relies on this, as by default matrices are plotted with the vertical axis directed downward. This leads to subtle issues like the following.

sage: p = matrix_plot(identity_matrix(5))
sage: p.get_minmax_data()
{ xmin:-0.5, ymin:4.5, ymax:-0.5, xmax:4.5 }
sage: p2 = p + point((2, 2), zorder=1)
sage: p2.get_minmax_data()
{ xmin:-0.5, ymin:1.0, ymax:3.0, xmax:4.5 }

Here, the bounding box of p has ymin > ymax which does not get taken into account when adding plots. As a consequence, p2 does not show the entire matrix.

As a solution that is hopefully more robust, this ticket adds flip_x/flip_y options that are documented to the user. For example:

sage: plot(x, -1, 1, flip_x=True)

From now on, the minmax data should always have min <= max, though for backward compatibility this is not enforced. When summing plots, flipped axes take precedence.

In matrix_plot, the now redundant option origin='upper'/origin='lower' is deprecated and replaced by flip_y.

Moreover, this ticket fixes an issue with plots of sparse matrices for which the data was incorrectly flipped:

sage: matrix_plot(identity_matrix(5, sparse=True), origin='lower')

See #18463 and #18612 for some related discussions.

Component: graphics

Keywords: matplotlib

Author: Markus Wageringel

Branch/Commit: 6553689

Reviewer: Vincent Delecroix

Issue created by migration from https://trac.sagemath.org/ticket/27891