plots: shorten names in the legend (#8900)

* plots: shorten field names

* plots: handle missing field names

* plots: make compatible with py3.8

* plots: fix for missing data

* plots: add test for field shortening

* plots: strip separators regardless of os

* plots: don't shorten field name

* plots: drop test for shortening field name

dvc/render/converter/vega.py

@@ -1,3 +1,4 @@
+import os
 from typing import Any, Dict, Iterable, List, Optional, Tuple, Union
 
 from funcy import first, last
@@ -221,7 +222,13 @@ class VegaConverter(Converter):
             )
 
         all_datapoints = []
-        all_y_fields = {y_field for _, y_field in ys}
+        if ys:
+            all_y_files, all_y_fields = list(zip(*ys))
+            all_y_fields = set(all_y_fields)
+            all_y_files = set(all_y_files)
+        else:
+            all_y_files = set()
+            all_y_fields = set()
 
         # override to unified y field name if there are different y fields
         if len(all_y_fields) > 1:
@@ -229,6 +236,12 @@ class VegaConverter(Converter):
         else:
             props_update["y"] = first(all_y_fields)
 
+        # get common prefix to drop from file names
+        if len(all_y_files) > 1:
+            common_prefix_len = len(os.path.commonpath(all_y_files))
+        else:
+            common_prefix_len = 0
+
         for i, (y_file, y_field) in enumerate(ys):
             if num_xs > 1:
                 x_file, x_field = xs[i]
@@ -258,12 +271,13 @@ class VegaConverter(Converter):
                         "They have to have same length."
                     )
 
+            y_file_short = y_file[common_prefix_len:].strip("/\\")
             _update_all(
                 datapoints,
                 update_dict={
                     VERSION_FIELD: {
                         "revision": revision,
-                        FILENAME_FIELD: y_file,
+                        FILENAME_FIELD: y_file_short,
                         "field": y_field,
                     }
                 },

tests/unit/render/test_vega_converter.py

@@ -379,6 +379,40 @@ def test_finding_lists(dictionary, expected_result):
             },
             id="multi_source_y_single_x",
         ),
+        pytest.param(
+            {
+                "dir/f": {"metric": [{"v": 1, "v2": 0.1}]},
+                "dir/f2": {"metric": [{"v": 1, "v2": 0.1}]},
+            },
+            {"y": {"dir/f": ["v2"], "dir/f2": ["v2"]}, "x": "v"},
+            [
+                {
+                    "v": 1,
+                    "v2": 0.1,
+                    VERSION_FIELD: {
+                        "revision": "r",
+                        "filename": "f",
+                        "field": "v2",
+                    },
+                },
+                {
+                    "v": 1,
+                    "v2": 0.1,
+                    VERSION_FIELD: {
+                        "revision": "r",
+                        "filename": "f2",
+                        "field": "v2",
+                    },
+                },
+            ],
+            {
+                "x": "v",
+                "y": "v2",
+                "x_label": "v",
+                "y_label": "v2",
+            },
+            id="multi_file_y_same_prefix",
+        ),
     ],
 )
 def test_convert(