fix(graphics): axes_logic uses an empty current figure instead of creating a second one

axes_logic reuses the current axes when the current figure has axes, but when
the current figure had none, as after a bare plt.figure(), it called
plt.figure() again.  Every "new plot" idiom (plt.figure() followed by a plot_xxx
call) therefore left an empty figure behind, most visible with the ipympl
backend where figures stay open in notebooks.

Put the axes in the empty current figure instead.  new=True still always makes a
new figure, a dimension mismatch still makes a new figure, and a current figure
that has axes is reused as before.

Add direct tests of axes_logic (there were none): the existing behaviour, and the
empty-current-figure cases.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Author: petercorke

spatialmath/base/graphics.py

@@ -1580,6 +1580,7 @@ def axes_logic(
         """
 
         # print(f"new axis logic ({dimensions}D): ", end='')
+        use_current_figure = False
         if ax is None:
             # no axes passed in, find out what's happening
             # need to be careful to not use gcf() or gca() since they
@@ -1597,6 +1598,10 @@ def axes_logic(
                     # print(f"ax has {_axes_dimensions(ax)} dimensions")
                     if _axes_dimensions(ax) == dimensions and not new:
                         return ax
+                elif not new:
+                    # the current figure is empty, eg. from a bare plt.figure():
+                    # put the axes in it rather than leave it empty and make another
+                    use_current_figure = True
             # otherwise it doesnt exist or dimension mismatch, create new axes
         else:
             # print("ax given", ax)
@@ -1607,7 +1612,8 @@ def axes_logic(
                 return ax
             # print("mismatch in dimensions, create new axes")
         # print("create new axes")
-        plt.figure()
+        if not use_current_figure:
+            plt.figure()
         if dimensions == 2:
             ax = plt.axes()
             if autoscale:

tests/base/test_axes_logic.py

@@ -0,0 +1,102 @@
+"""Tests of ``axes_logic``, which decides which axes a plot_xxx() function draws into."""
+
+import matplotlib
+
+matplotlib.use("Agg")
+import matplotlib.pyplot as plt
+import pytest
+
+from spatialmath.base.graphics import axes_logic
+
+
+@pytest.fixture(autouse=True)
+def close_figures():
+    plt.close("all")
+    yield
+    plt.close("all")
+
+
+def nfigs():
+    return len(plt.get_fignums())
+
+
+# --- behaviour that must not change ---------------------------------------------
+
+
+def test_no_figure_creates_one():
+    ax = axes_logic(None, 2)
+    assert nfigs() == 1
+    assert ax is plt.gca()
+
+
+def test_current_axes_are_reused():
+    fig, ax0 = plt.subplots()
+    ax = axes_logic(None, 2)
+    assert ax is ax0
+    assert nfigs() == 1
+
+
+def test_new_forces_a_new_figure():
+    fig, ax0 = plt.subplots()
+    ax = axes_logic(None, 2, new=True)
+    assert ax is not ax0
+    assert nfigs() == 2
+    assert ax is plt.gca()
+
+
+def test_dimension_mismatch_makes_a_new_figure():
+    fig = plt.figure()
+    ax3 = fig.add_subplot(projection="3d")
+    ax = axes_logic(None, 2)
+    assert ax is not ax3
+    assert nfigs() == 2
+
+
+def test_explicit_axes_of_the_right_dimension_are_used():
+    fig, ax0 = plt.subplots()
+    fig2, ax1 = plt.subplots()
+    assert axes_logic(ax0, 2) is ax0
+    assert nfigs() == 2
+
+
+def test_explicit_axes_of_the_wrong_dimension_make_a_new_figure():
+    fig, ax0 = plt.subplots()
+    ax = axes_logic(ax0, 3)
+    assert ax is not ax0
+    assert nfigs() == 2
+
+
+# --- an empty current figure, eg. from a bare plt.figure() -----------------------
+
+
+def test_empty_current_figure_is_used_2d():
+    fig = plt.figure()
+    ax = axes_logic(None, 2)
+    assert nfigs() == 1
+    assert ax.figure is fig
+    assert ax is plt.gca()
+
+
+def test_empty_current_figure_is_used_3d():
+    fig = plt.figure()
+    ax = axes_logic(None, 3)
+    assert nfigs() == 1
+    assert ax.figure is fig
+    assert ax.name == "3d"
+
+
+def test_only_the_current_figure_matters():
+    # an earlier figure has axes, but the current one is empty: use the current one
+    plt.subplots()
+    fig = plt.figure()
+    ax = axes_logic(None, 2)
+    assert nfigs() == 2
+    assert ax.figure is fig
+
+
+def test_new_with_an_empty_current_figure_still_makes_another():
+    # new=True is an explicit request for a new figure
+    fig = plt.figure()
+    ax = axes_logic(None, 2, new=True)
+    assert nfigs() == 2
+    assert ax.figure is not fig