feat(plot): configure one or two marker pairs per axis

python/tests/test_plot.py

@@ -5,6 +5,7 @@ import math
 import os
 from pathlib import Path
 import unittest
+from dataclasses import replace
 
 from protocan.plot import Bounds, Marker, Markers, PlotMath, Viewport
 
@@ -31,6 +32,9 @@ class PlotTests(unittest.TestCase):
     def test_markers_stay_in_data_coordinates_and_cross(self):
         bounds = Bounds(1000, 2000, -10, 10)
         markers = Markers().positioned(self.core, bounds)
+        self.assertTrue(markers.enabled(Marker.A))
+        self.assertFalse(markers.enabled(Marker.C))
+        self.assertTrue(replace(markers, x_pairs=2).enabled(Marker.C))
         zoomed = bounds.visible(self.core, Viewport(.25, .25, .5, .5))
         self.assertEqual(markers, markers.positioned(self.core, zoomed))
         moved = markers.drag(self.core, Marker.A, 50, 500, zoomed)