diff --git a/dvc/repo/plots/template.py b/dvc/repo/plots/template.py
--- a/dvc/repo/plots/template.py
+++ b/dvc/repo/plots/template.py
@@ -12,6 +12,10 @@ def __init__(self, path):
         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 @@ def __init__(self, content=None, name=None):
     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"):

Page source:

```html

<!DOCTYPE html>
<html>
<head>
    <title>DVC Plot</title>
    <script src="https://cdn.jsdelivr.net/npm/vega@5.10.0"></script>
    <script src="https://cdn.jsdelivr.net/npm/vega-lite@4.8.1"></script>
    <script src="https://cdn.jsdelivr.net/npm/vega-embed@6.5.1"></script>
</head>
<body>
    <div id = "plot0"></div>
<script type = "text/javascript">
    var spec = epoch,accuracy,loss
0,0.9403833150863647,0.2019129991531372
1,0.9733833074569702,0.08973673731088638
2,0.9815833568572998,0.06529958546161652
3,0.9861999750137329,0.04984375461935997
4,0.9882333278656006,0.041892342269420624
;
    vegaEmbed('#plot0', spec);
</script>
</body>
</html>
```
That's pretty strange, works for me just fine:
```
<!DOCTYPE html>
--
  | <html>
  | <head>
  | <title>DVC Plot</title>
  | <script src="https://cdn.jsdelivr.net/npm/vega@5.10.0"></script>
  | <script src="https://cdn.jsdelivr.net/npm/vega-lite@4.8.1"></script>
  | <script src="https://cdn.jsdelivr.net/npm/vega-embed@6.5.1"></script>
  | </head>
  | <body>
  | <div id = "plot0"></div>
  | <script type = "text/javascript">
  | var spec = {
  | "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  | "data": {
  | "values": [
  | {
  | "accuracy": "0.9403833150863647",
  | "epoch": "0",
  | "index": 0,
  | "loss": "0.2019129991531372",
  | "rev": "workspace"
  | },
  | {
  | "accuracy": "0.9733833074569702",
  | "epoch": "1",
  | "index": 1,
  | "loss": "0.08973673731088638",
  | "rev": "workspace"
  | },
  | {
  | "accuracy": "0.9815833568572998",
  | "epoch": "2",
  | "index": 2,
  | "loss": "0.06529958546161652",
  | "rev": "workspace"
  | },
  | {
  | "accuracy": "0.9861999750137329",
  | "epoch": "3",
  | "index": 3,
  | "loss": "0.04984375461935997",
  | "rev": "workspace"
  | },
  | {
  | "accuracy": "0.9882333278656006",
  | "epoch": "4",
  | "index": 4,
  | "loss": "0.041892342269420624",
  | "rev": "workspace"
  | }
  | ]
  | },
  | "title": "",
  | "mark": {
  | "type": "line"
  | },
  | "encoding": {
  | "x": {
  | "field": "index",
  | "type": "quantitative",
  | "title": "index"
  | },
  | "y": {
  | "field": "loss",
  | "type": "quantitative",
  | "title": "loss",
  | "scale": {
  | "zero": false
  | }
  | },
  | "color": {
  | "field": "rev",
  | "type": "nominal"
  | }
  | }
  | }
  | ;
  | vegaEmbed('#plot0', spec);
  | </script>
  | </body>
  | </html>
```
@jorgeorpinel did you use standard templates?
> ```
> <!DOCTYPE html>
> --
>  | <html>
>  | <head>
> ...
> ```

Tehehe copy paste from Chrome "view source" introduces a bunch of characters (works properly with ctrl+shift+v for me).
> did you use standard templates?

🤦 ! I mistakenly used the `-t train.json` flag thinking it meant `--targets` but it means `--template` ! `dvc plots diff --targets train.json` works.
Maybe we should ~~reconsider the option letters? Or at least~~ print a warning when a provided template file does not exist or is invalid! Cc @pared Thanks!
@jorgeorpinel It already throws if the template is not found:

```
(dvc-3.8.3) ➜  dvc git:(fix-3897) ✗ dvc plots diff -t sdf
ERROR: Template 'sdf' not found.

Having any troubles? Hit us up at https://dvc.org/support, we are always happy to help!
```

It is not able to detect a bad template, that is true, but I'm not completely sure if we can do that in a general case. Seems like the best we could do is to check that it is indeed a json/yaml (once we get rid of html support), but I'm not sure we could require it to hit any quotas on anchors that it uses.  CC @pared  
> not able to detect a bad template, that is true, but I'm not completely sure if we can do that in a general case

Maybe let's just keep an eye on this to see if users make the same error I did and at least detect when a given template is actually a metrics file in that case (and WARN/ERROR out).
We can check that template is valid by checking for `DVC_METRIC_DATA` anchor. I don't see a use case where one would like to use `dvc plots` and not provide it. 

Also if we are talking about JSON templates, what is a common property of a JSON specification is `$schema` field.
eg
- for vega:
`"$schema": "https://vega.github.io/schema/vega/v5.json"`
- vega lite:
` "$schema": "https://vega.github.io/schema/vega-lite/v4.json"`
It can even be done by [JSON schema validation](https://datatracker.ietf.org/doc/draft-handrews-json-schema-validation/) but seems overkill (unless there's a quick python lib that does easily).
Taking a quick peek at available resources, https://github.com/Julian/jsonschema seems like the most popular python-based and maintained package.

The question is whether we want to include a new package for one functionality validation.

 If we do, it might help us solving `plot update` (if we decide to do it) from https://github.com/iterative/dvc/issues/3906 

I think it would make sense.
@pared So looks like our templates have the schema already, which validates the generated template, right? So we are covered there. The only thing that we need to validate is that the template is indeed using our anchors.
@efiop yep, I think this would solve this issue.
diff --git a/tests/func/plots/test_plots.py b/tests/func/plots/test_plots.py
--- a/tests/func/plots/test_plots.py
+++ b/tests/func/plots/test_plots.py
@@ -16,7 +16,11 @@
     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(

plots diff: WARN about bad templates sent to -t [qa]
## ~~Bug Report~~

UPDATE: Jump to https://github.com/iterative/dvc/issues/3969#issuecomment-640879725

---
OLD:

Using logs.csv from the example in https://dvc-landing-2020-05-31-evbiu1f.herokuapp.com/doc/command-reference/plots/modify#examples (see iterative/dvc.org/pull/1382):

```console
dvc plots diff -t train.json
file://C:\Users\poj12\DVC-repos\tests\plots.html
```

![image](https://user-images.githubusercontent.com/1477535/83983047-c3ded700-a8f0-11ea-90e7-d3db24a21916.png)

### Please provide information about your setup

**Output of `dvc version`:**

```console
λ dvc version
DVC version: 1.0.0a9+3683c2
Python version: 3.8.2
Platform: Windows-10-10.0.18362-SP0
Binary: False
Package: None
Supported remotes: azure, gdrive, gs, hdfs, http, https, s3, ssh, oss
Cache: reflink - not supported, hardlink - supported, symlink - supported
Filesystem type (cache directory): ('NTFS', 'C:\\')
Repo: dvc, git
Filesystem type (workspace): ('NTFS', 'C:\\')
```

**Additional Information (if any):**

If applicable, please also provide a `--verbose` output of the command, eg: `dvc add --verbose`.


tests/func/plots/test_plots.py::test_plot_csv_one_column tests/func/plots/test_plots.py::test_plot_csv_multiple_columns tests/func/plots/test_plots.py::test_plot_csv_choose_axes tests/func/plots/test_plots.py::test_plot_json_single_val tests/func/plots/test_plots.py::test_plot_json_multiple_val tests/func/plots/test_plots.py::test_plot_confusion tests/func/plots/test_plots.py::test_plot_multiple_revs_default tests/func/plots/test_plots.py::test_plot_multiple_revs tests/func/plots/test_plots.py::test_plot_even_if_metric_missing tests/func/plots/test_plots.py::test_throw_on_no_metric_at_all tests/func/plots/test_plots.py::test_custom_template tests/func/plots/test_plots.py::test_no_plots tests/func/plots/test_plots.py::test_should_raise_on_no_template tests/func/plots/test_plots.py::test_bad_template tests/func/plots/test_plots.py::test_plot_wrong_metric_type tests/func/plots/test_plots.py::test_plot_choose_columns tests/func/plots/test_plots.py::test_plot_default_choose_column tests/func/plots/test_plots.py::test_plot_yaml tests/func/plots/test_plots.py::test_raise_on_wrong_field tests/func/plots/test_plots.py::test_load_metric_from_dict_json tests/func/plots/test_plots.py::test_load_metric_from_dict_yaml tests/func/plots/test_plots.py::test_multiple_plots