diff --git a/dvc/render/converter/vega.py b/dvc/render/converter/vega.py
index e723253f5..e9c302f7f 100644
--- a/dvc/render/converter/vega.py
+++ b/dvc/render/converter/vega.py
@@ -181,6 +181,8 @@ class VegaConverter(Converter):
                 fields = {field for _, field in _file_field(x)}
                 if len(fields) == 1:
                     x_label = first(fields)
+                else:
+                    x_label = "x"
             else:
                 x_label = INDEX_FIELD
         return x_label