plots: fix error when there are no plots in repo (#3940)

Consistent with `dvc metrics show/diff` behaviour.

Fixes #3925

dvc/exceptions.py

@@ -178,6 +178,14 @@ class NoMetricsError(DvcException):
         )
 
 
+class NoPlotsError(DvcException):
+    def __init__(self):
+        super().__init__(
+            "no plots in this repository. Use `--plots/--plots-no-cache` "
+            "options for `dvc run` to mark stage outputs as plots."
+        )
+
+
 class YAMLFileCorruptedError(DvcException):
     def __init__(self, path):
         path = relpath(path)

dvc/repo/plots/show.py

@@ -4,7 +4,7 @@ import os
 
 from funcy import first, last, project
 
-from dvc.exceptions import DvcException
+from dvc.exceptions import DvcException, NoPlotsError
 from dvc.repo import locked
 from dvc.schema import PLOT_PROPS
 
@@ -108,6 +108,9 @@ def _show(repo, datafile=None, revs=None, props=None):
     if revs is None:
         revs = ["workspace"]
 
+    if props is None:
+        props = {}
+
     if not datafile and not props.get("template"):
         raise NoDataOrTemplateProvided()
 
@@ -201,7 +204,11 @@ def show(repo, targets=None, revs=None, props=None) -> dict:
         if targets:
             raise NoMetricInHistoryError(", ".join(targets))
 
-        datafile, plot = _show(repo, datafile=None, revs=revs, props=props)
+        try:
+            datafile, plot = _show(repo, datafile=None, revs=revs, props=props)
+        except NoDataOrTemplateProvided:
+            raise NoPlotsError()
+
         return {datafile: plot}
 
     return {

tests/func/plots/test_plots.py

@@ -14,7 +14,6 @@ from dvc.repo.plots.data import (
     PlotData,
     PlotMetricTypeError,
 )
-from dvc.repo.plots.show import NoDataOrTemplateProvided
 from dvc.repo.plots.template import (
     NoDataForTemplateError,
     NoFieldInDataError,
@@ -457,8 +456,10 @@ def test_plot_override_specified_data_source(
     assert plot_content["encoding"]["y"]["field"] == "b"
 
 
-def test_should_raise_on_no_template_and_datafile(tmp_dir, dvc):
-    with pytest.raises(NoDataOrTemplateProvided):
+def test_no_plots(tmp_dir, dvc):
+    from dvc.exceptions import NoPlotsError
+
+    with pytest.raises(NoPlotsError):
         dvc.plots.show()