Hide crosshair lines from the curve options widget to avoid errors (#39611)

We found a bug with the new crosshair plotting feature where its lines
would show up in the plot settings (see comment
#39598 (comment))

This pr gives the crosshair lines the label `'_nolegend_'` which means
they are ignored by the plot settings.


Co-authored-by: thomashampson <thomas.hampson@stfc.ac.uk>

Author: jhaigh0

qt/applications/workbench/workbench/plotting/figuremanager.py

@@ -611,8 +611,8 @@ def crosshair_toggle(self, on):
             for ax in self._axes_that_are_not_colour_bars():
                 ax.set_autoscalex_on(False)
                 ax.set_autoscaley_on(False)
-                hline = ax.axhline(color="r", lw=1.0, ls="-", visible=False)
-                vline = ax.axvline(color="r", lw=1.0, ls="-", visible=False)
+                hline = ax.axhline(color="r", lw=1.0, ls="-", visible=False, label="_nolegend_")
+                vline = ax.axvline(color="r", lw=1.0, ls="-", visible=False, label="_nolegend_")
                 self._crosshair_lines[ax] = (hline, vline)
 
             self._crosshair_cid = self.canvas.mpl_connect("motion_notify_event", self.crosshair)

qt/python/mantidqt/mantidqt/widgets/plotconfigdialog/__init__.py

@@ -86,8 +86,15 @@ def curve_in_ax(ax):
 
 
 def line_in_ax(ax):
-    """Return True if there are any lines in the Axes object"""
-    return len(ax.get_lines()) > 0
+    """Return True if there are any lines in the Axes object, ignoring any lines with the _nolegend_ label (e.g. the crosshair)."""
+
+    def safe_get_label(line):
+        if hasattr(line, "get_label"):
+            return line.get_label()
+        return ""
+
+    filtered_lines = [line for line in ax.get_lines() if not safe_get_label(line) == "_nolegend_"]
+    return len(filtered_lines) > 0
 
 
 def errorbars_in_ax(ax):