Fix barrier label trunctation in matplotlib circuit renders (#15973)

* Fix mpl barrier_label_len with non-default value

* update test

* fix the whitespace issue too

* update test image

Author: ihincks

qiskit/visualization/circuit/circuit_visualization.py

@@ -382,6 +382,7 @@ def check_clbit_in_inst(circuit, cregbundle):
             wire_order=complete_wire_order,
             expr_len=expr_len,
             measure_arrows=measure_arrows,
+            barrier_label_len=barrier_label_len,
         )
     else:
         raise VisualizationError(

qiskit/visualization/circuit/matplotlib.py

@@ -431,6 +431,10 @@ def _get_layer_widths(self, node_data, wire_map, outer_circuit, glob_data):
                 gate_text, ctrl_text, raw_gate_text = get_gate_ctrl_text(
                     op, "mpl", style=self._style
                 )
+                # Truncate directive labels to match the rendered length so the layer
+                # width doesn't account for text that will never be displayed.
+                if getattr(op, "_directive", False) and len(gate_text) > self._barrier_label_len:
+                    gate_text = gate_text[: self._barrier_label_len] + "..."
                 node_data[node].gate_text = gate_text
                 node_data[node].ctrl_text = ctrl_text
                 # Measure doesn't use raw_gate_text since it displays a dial

test/visual/mpl/circuit/references/barrier_label_truncation.png

@@ -1246,7 +1246,8 @@ def test_barrier_label_truncation(self):
         """Test that long barrier labels are truncated"""
         circuit = QuantumCircuit(2)
         circuit.barrier()
-        circuit.barrier(label="a" * 20)
+        circuit.barrier(label="a" * 10)
+        circuit.barrier(label="b" * 1000)
 
         fname = "barrier_label_truncation.png"
         self.circuit_drawer(circuit, output="mpl", filename=fname, barrier_label_len=9)