Fixed the positioning of line graph that was under the graph (#1951)

* removed a part of code that positioned a new line graph behind the axes

* added a line graph graphical unit test

Co-authored-by: Hugues Devimeux <[email protected]>

Author: vasek123

manim/mobject/coordinate_systems.py

@@ -1255,7 +1255,6 @@ def construct(self):
             for x, y, z in zip(x_values, y_values, z_values)
         ]
         graph.set_points_as_corners(vertices)
-        graph.z_index = -1
         line_graph["line_graph"] = graph
 
         if add_vertex_dots:

tests/control_data/graphical_units_data/coordinate_system/line_graph.npz

@@ -16,3 +16,16 @@ def test_number_plane(scene):
     )
 
     scene.add(plane)
+
+
+@frames_comparison
+def test_line_graph(scene):
+    plane = NumberPlane()
+    first_line = plane.get_line_graph(
+        x_values=[-3, 1], y_values=[-2, 2], line_color=YELLOW
+    )
+    second_line = plane.get_line_graph(
+        x_values=[0, 2, 2, 4], y_values=[0, 0, 2, 4], line_color=RED
+    )
+
+    scene.add(plane, first_line, second_line)