plots: make sure template is using data anchor (#4045)

Fixes #3969

dvc/repo/plots/template.py

@@ -12,6 +12,10 @@ class TemplateNotFoundError(DvcException):
         super().__init__(f"Template '{path}' not found.")
 
 
+class BadTemplateError(DvcException):
+    pass
+
+
 class NoFieldInDataError(DvcException):
     def __init__(self, field_name):
         super().__init__(
@@ -39,6 +43,12 @@ class Template:
     def render(self, data, props=None):
         props = props or {}
 
+        if self.METRIC_DATA_ANCHOR not in self.content:
+            raise BadTemplateError(
+                f"Template '{self.filename}' is not using "
+                f"'{self.METRIC_DATA_ANCHOR}' anchor"
+            )
+
         if props.get("x"):
             Template._check_field_exists(data, props.get("x"))
         if props.get("y"):

tests/func/plots/test_plots.py

@@ -16,7 +16,11 @@ from dvc.repo.plots.data import (
     PlotMetricTypeError,
     YAMLPlotData,
 )
-from dvc.repo.plots.template import NoFieldInDataError, TemplateNotFoundError
+from dvc.repo.plots.template import (
+    BadTemplateError,
+    NoFieldInDataError,
+    TemplateNotFoundError,
+)
 
 
 def _write_csv(metric, filename, header=True):
@@ -405,6 +409,23 @@ def test_should_raise_on_no_template(tmp_dir, dvc, run_copy_metrics):
         dvc.plots.show("metric.json", props=props)
 
 
+def test_bad_template(tmp_dir, dvc, run_copy_metrics):
+    metric = [{"val": 2}, {"val": 3}]
+    _write_json(tmp_dir, metric, "metric_t.json")
+    run_copy_metrics(
+        "metric_t.json",
+        "metric.json",
+        plots_no_cache=["metric.json"],
+        commit="first run",
+    )
+
+    tmp_dir.gen("template.json", json.dumps({"a": "b", "c": "d"}))
+
+    with pytest.raises(BadTemplateError):
+        props = {"template": "template.json"}
+        dvc.plots.show("metric.json", props=props)
+
+
 def test_plot_wrong_metric_type(tmp_dir, scm, dvc, run_copy_metrics):
     tmp_dir.gen("metric_t.txt", "some text")
     run_copy_metrics(