diff --git a/README.md b/README.md
index 375370b..47ea95b 100644
--- a/README.md
+++ b/README.md
@@ -12,11 +12,12 @@ The figure shows the trajectory in the plane, `f(mu) = psi(x(mu))`, the distance
 ## Usage
 
     pip install -r requirements.txt
-    python plot_mu_trajectories.py                 # degenerate case x0 = (0, -0.1)
-    python plot_mu_trajectories.py --x0 0.3 0.2 --output figures/generic
+    python plot_mu_trajectories.py                         # every case, into figures/
+    python plot_mu_trajectories.py x_trajectories_mu_2     # a single case
     pytest
 
-The ellipse is set by `A`, `B`, `C` at the top of `plot_mu_trajectories.py`.
+Cases (`A`, `x0`, and optionally `b` and `c`) are listed in `CASES` at the top of
+`plot_mu_trajectories.py`; each one produces `<name>.png` and `<name>.pdf`.
 
 Text is typeset with LaTeX (`text.usetex`) when `latex` is on the `PATH`, with the `cm-super` and
 `type1cm` packages; otherwise matplotlib's Computer Modern math font is used.
diff --git a/plot_mu_trajectories.py b/plot_mu_trajectories.py
index 9ee09f8..c47eb45 100644
--- a/plot_mu_trajectories.py
+++ b/plot_mu_trajectories.py
@@ -19,9 +19,11 @@ from matplotlib.lines import Line2D
 
 from quadric import TOL, Ellipsoid
 
-A = np.array([[2.0, 0.0], [0.0, 1.0]])
-B = np.zeros(2)
-C = -1.0
+# One figure per case, for the ellipse x'Ax + b'x + c = 0 (b = 0 and c = -1 by default).
+CASES = {
+    'x_trajectories_mu_1': dict(A=np.diag([2.0, 1.0]), x0=[0.0, -0.1]),
+    'x_trajectories_mu_2': dict(A=np.diag([2.0, 0.75]), x0=[0.0, -0.6]),
+}
 
 N_PER_BRANCH = 266    # mu samples between consecutive poles
 EPS = 0.01            # gap left on each side of a pole
@@ -163,16 +165,23 @@ def use_latex_fonts():
 def main():
     use_latex_fonts()
     parser = argparse.ArgumentParser(description=__doc__.splitlines()[0])
-    parser.add_argument('--x0', type=float, nargs=2, default=[0.0, -0.1], metavar=('X1', 'X2'))
-    parser.add_argument('--output', type=Path, default=Path('figures/x_trajectories_mu'),
-                        help='output path, without extension (a .png and a .pdf are written)')
+    parser.add_argument('cases', nargs='*', metavar='CASE',
+                        help=f'cases to plot, among {", ".join(CASES)} (default: all)')
+    parser.add_argument('--output-dir', type=Path, default=Path('figures'))
     parser.add_argument('--show', action='store_true')
     args = parser.parse_args()
+    unknown = set(args.cases) - set(CASES)
+    if unknown:
+        parser.error(f'unknown cases: {", ".join(sorted(unknown))}')
 
-    fig = plot_trajectories(Ellipsoid(A, B, C), np.array(args.x0))
-    args.output.parent.mkdir(parents=True, exist_ok=True)
-    for extension in ('.png', '.pdf'):
-        fig.savefig(args.output.with_name(args.output.name + extension))
+    args.output_dir.mkdir(parents=True, exist_ok=True)
+    for name in args.cases or CASES:
+        case = CASES[name]
+        Q = Ellipsoid(case['A'], case.get('b', np.zeros(2)), case.get('c', -1.0))
+        fig = plot_trajectories(Q, np.array(case['x0']))
+        for extension in ('.png', '.pdf'):
+            fig.savefig(args.output_dir / (name + extension))
+        print('wrote', args.output_dir / name)
     if args.show:
         plt.show()