Cycle only over filled markers for default artist style

src/plopp/backends/matplotlib/line.py

@@ -9,12 +9,11 @@
 import scipp as sc
 from matplotlib.axes import Axes
 from matplotlib.dates import date2num
-from matplotlib.lines import Line2D
 
 from ...graphics.bbox import BoundingBox
 from ..common import check_ndim, make_line_bbox, make_line_data
 from .canvas import Canvas
-from .utils import parse_dicts_in_kwargs
+from .utils import default_marker, parse_dicts_in_kwargs
 
 
 def _to_float(x):
@@ -255,11 +254,10 @@ def __init__(
             'color': f'C{artist_number}',
             'zorder': 2,
         }
-        markers = list(Line2D.markers.keys())
         default_plot_style = {
             'linestyle': 'none',
             'linewidth': 1.5,
-            'marker': markers[(artist_number + 2) % len(markers)],
+            'marker': default_marker(artist_number),
             'color': f'C{artist_number}',
             'zorder': 2,
         }

src/plopp/backends/matplotlib/scatter.py

@@ -6,14 +6,13 @@
 
 import numpy as np
 import scipp as sc
-from matplotlib.lines import Line2D
 
 from ...core.utils import merge_masks
 from ...graphics.bbox import BoundingBox, axis_bounds
 from ...graphics.colormapper import ColorMapper
 from ..common import check_ndim
 from .canvas import Canvas
-from .utils import parse_dicts_in_kwargs
+from .utils import default_marker, parse_dicts_in_kwargs
 
 
 class Scatter:
@@ -79,9 +78,8 @@ def __init__(
         self._unit = self._data.unit
         self._id = uuid.uuid4().hex
 
-        markers = list(Line2D.markers.keys())
         default_plot_style = {
-            'marker': markers[(artist_number + 2) % len(markers)],
+            'marker': default_marker(artist_number),
         }
         if not cbar:
             default_plot_style['color'] = f'C{artist_number}'

src/plopp/backends/matplotlib/utils.py

@@ -6,6 +6,7 @@
 
 import matplotlib as mpl
 import matplotlib.pyplot as plt
+from matplotlib.lines import Line2D
 
 
 def fig_to_bytes(fig: plt.Figure, form: Literal['png', 'svg'] = 'png') -> bytes:
@@ -72,6 +73,18 @@ def make_figure(*args, **kwargs) -> plt.Figure:
     return fig
 
 
+def default_marker(artist_number: int) -> str:
+    """
+    Return a marker from a cycle of markers, based on the artist number.
+
+    Only filled markers are used, as ``Line2D.markers`` also contains integer markers
+    and markers that draw nothing (e.g. ``'none'`` and ``''``).
+    """
+    markers = Line2D.filled_markers
+    # Start the cycle at 'o' instead of the barely visible '.'.
+    return markers[(artist_number + 1) % len(markers)]
+
+
 def make_legend(leg: bool | tuple[float, float] | str) -> dict:
     """
     Create a dict of arguments to be used in the legend creation.

tests/backends/matplotlib/mpl_line_test.py

@@ -4,6 +4,7 @@
 import numpy as np
 import pytest
 import scipp as sc
+from matplotlib.markers import MarkerStyle
 
 from plopp.backends.matplotlib.canvas import Canvas
 from plopp.backends.matplotlib.line import Line
@@ -183,6 +184,14 @@ def test_kwarg_marker():
     assert line._line.get_marker() == '+'
 
 
+def test_default_marker_is_visible_for_large_artist_number():
+    da = data_array(ndim=1)
+    canvas = Canvas()
+    for artist_number in range(50):
+        line = Line(canvas=canvas, data=da, artist_number=artist_number)
+        assert MarkerStyle(line.marker).is_filled()
+
+
 @pytest.mark.parametrize("mode", ['band', 'bar', True])
 def test_line_color_with_errorbars(mode):
     from matplotlib.colors import to_hex

tests/backends/matplotlib/mpl_plot_test.py

@@ -202,3 +202,9 @@ def test_aspect_ratio():
     da = data_array(ndim=2)
     fig = da.plot(aspect='equal')
     assert fig.canvas.ax.get_aspect() == 1.0
+
+
+def test_plot_many_lines():
+    da = data_array(ndim=1)
+    fig = pp.plot({str(i): da for i in range(50)})
+    assert len(fig.artists) == 50