bench: add test_plots_diff

tests/benchmarks/cli/commands/test_plots.py

@@ -0,0 +1,42 @@
+from textwrap import dedent
+
+import pytest
+
+
+@pytest.fixture
+def repo_with_plots(tmp_dir, scm, dvc):
+    num_points = 1000
+    num_files = 50
+
+    CODE = dedent(
+        """
+        import json
+        import sys
+        num_points=int(sys.argv[1])
+        num_files=int(sys.argv[2])
+        metric = [{'m':(i/num_points)**2} for i in range(0, num_points)]
+        for i in range(num_files):
+            with open(f'metric_{i}.json', 'w') as fd:
+                json.dump(metric, fd)
+        """
+    )
+
+    tmp_dir.gen("train.py", CODE)
+
+    dvc.run(
+        name=f"generate_plots",
+        deps=["train.py"],
+        plots=[f"metric_{i}.json" for i in range(num_files)],
+        cmd=f"python train.py {num_points} {num_files}",
+    )
+
+    scm.add(["dvc.lock", "dvc.yaml"])
+    scm.commit("initial run")
+
+    yield
+
+
+def test_plots_diff(tmp_dir, bench_dvc, repo_with_plots):
+    bench_dvc("plots", "show")
+
+    assert (tmp_dir / "dvc_plots" / "index.html").is_file()