diff --git a/plotter.py b/plotter.py
index 9e483a7..da08c0d 100644
--- a/plotter.py
+++ b/plotter.py
@@ -1001,12 +1001,12 @@ class Plotter(Aggregator, BaseProcessor):
         if s is None and sinks:
             s = data.msink[mask] / 5e3
 
-        if isinstance(s, str):
+        if s in data.keys():
             s = data[s][mask]
         elif callable(s):
             s = s(data)[mask]
 
-        if isinstance(c, str):
+        if c in data.keys():
             c = data[c][mask]
         elif callable(c):
             c = c(data)[mask]