tests: plots: add simple diff test

tests/func/plots/test_diff.py

@@ -0,0 +1,29 @@
+import json
+
+from .test_plots import PlotData, _run_with, _write_json
+
+
+def test_diff_dirty(tmp_dir, scm, dvc):
+    metric_1 = [{"y": 2}, {"y": 3}]
+    _write_json(tmp_dir, metric_1, "metric.json")
+    _run_with(tmp_dir, plots_no_cache=["metric.json"], commit="init")
+
+    metric_2 = [{"y": 3}, {"y": 5}]
+    _write_json(tmp_dir, metric_2, "metric.json")
+    _run_with(tmp_dir, plots_no_cache=["metric.json"], commit="second")
+
+    metric_3 = [{"y": 5}, {"y": 6}]
+    _write_json(tmp_dir, metric_3, "metric.json")
+    _run_with(tmp_dir, plots_no_cache=["metric.json"])
+
+    plot_string = dvc.plots.diff(fields={"y"})["metric.json"]
+
+    plot_content = json.loads(plot_string)
+    assert plot_content["data"]["values"] == [
+        {"y": 5, PlotData.INDEX_FIELD: 0, "rev": "working tree"},
+        {"y": 6, PlotData.INDEX_FIELD: 1, "rev": "working tree"},
+        {"y": 3, PlotData.INDEX_FIELD: 0, "rev": "HEAD"},
+        {"y": 5, PlotData.INDEX_FIELD: 1, "rev": "HEAD"},
+    ]
+    assert plot_content["encoding"]["x"]["field"] == PlotData.INDEX_FIELD
+    assert plot_content["encoding"]["y"]["field"] == "y"