special case for plot_prune_fifo()

Author: prjemian

apstools/utils.py

@@ -1022,7 +1022,9 @@ def plot_prune_fifo(bec, n, y, x):
         lines = [
             tr
             for tr in lp.ax.lines
-            if len(tr._x) != 2 or len(tr._y) != 2 or tr._x[0] != tr._x[1]
+            if len(tr._x) != 2 
+		or len(tr._y) != 2 
+		or (len(tr._x) == 2 and tr._x[0] != tr._x[1])
         ]
         if len(lines) > n:
             logging.debug(f"limiting LivePlot({y.name}) to {n} traces")