Matrix plots have the wrong aspect ratio, causing the plot to collapse into a line. Compare:

sage: matrix_plot(identity_matrix(3))
sage: matrix_plot(identity_matrix(3), aspect_ratio='automatic')

See https://groups.google.com/d/msg/sage-support/mDaQWuqi9bg/g571Ij8_C8kJ.

CC: @kcrisman

Component: graphics

Author: David Einstein

Branch: 3ef07b4

Reviewer: William Stein

Issue created by migration from https://trac.sagemath.org/ticket/18463