Refactor vega-related charting (#8595)

## Describe your changes

This PR refactors/restructures our backend logic that handles all
vega-related charting: `st.vega_lite_chart`, `st.altair_chart`,
`st.line_chart`, `st.area_chart`, `st.bar_chart`, `st.scatter_chart`.

- The built-in charting logic is migrated to `built_in_chart_utils` 
- All commands are migrated to `vega_charts` and refactored to reuse
more shared logic.
- This also applies some refactoring to how we handle adding `add_rows`
metadata by just relaying on the available metadata instead of checking
for the delta type.

## Testing Plan

- Updated unit tests
- No big logical changes.

---

**Contribution License Agreement**

By submitting this pull request you agree that all contributions to this
project are made under the Apache 2.0 license.

lib/streamlit/delta_generator.py

@@ -46,10 +46,7 @@
 )
 from streamlit.cursor import Cursor
 from streamlit.elements.alert import AlertMixin
-from streamlit.elements.altair_utils import AddRowsMetadata
 from streamlit.elements.arrow import ArrowMixin
-from streamlit.elements.arrow_altair import ArrowAltairMixin, prep_data
-from streamlit.elements.arrow_vega_lite import ArrowVegaLiteMixin
 from streamlit.elements.balloons import BalloonsMixin
 from streamlit.elements.bokeh_chart import BokehMixin
 from streamlit.elements.code import CodeMixin
@@ -75,6 +72,7 @@
 from streamlit.elements.snow import SnowMixin
 from streamlit.elements.text import TextMixin
 from streamlit.elements.toast import ToastMixin
+from streamlit.elements.vega_charts import VegaChartsMixin
 from streamlit.elements.widgets.button import ButtonMixin
 from streamlit.elements.widgets.camera_input import CameraInputMixin
 from streamlit.elements.widgets.chat import ChatMixin
@@ -94,7 +92,7 @@
 from streamlit.errors import NoSessionContext, StreamlitAPIException
 from streamlit.proto import Block_pb2, ForwardMsg_pb2
 from streamlit.proto.RootContainer_pb2 import RootContainer
-from streamlit.runtime import caching, legacy_caching
+from streamlit.runtime import caching
 from streamlit.runtime.scriptrunner import get_script_run_ctx
 from streamlit.runtime.state import NoValue
 
@@ -104,19 +102,11 @@
     from pandas import DataFrame, Series
 
     from streamlit.elements.arrow import Data
+    from streamlit.elements.lib.built_in_chart_utils import AddRowsMetadata
 
 
 MAX_DELTA_BYTES: Final[int] = 14 * 1024 * 1024  # 14MB
 
-# List of Streamlit commands that perform a Pandas "melt" operation on
-# input dataframes:
-ARROW_DELTA_TYPES_THAT_MELT_DATAFRAMES: Final = (
-    "arrow_line_chart",
-    "arrow_area_chart",
-    "arrow_bar_chart",
-    "arrow_scatter_chart",
-)
-
 Value = TypeVar("Value")
 DG = TypeVar("DG", bound="DeltaGenerator")
 
@@ -197,8 +187,7 @@ class DeltaGenerator(
     ToastMixin,
     WriteMixin,
     ArrowMixin,
-    ArrowAltairMixin,
-    ArrowVegaLiteMixin,
+    VegaChartsMixin,
     DataEditorMixin,
 ):
     """Creator of Delta protobuf messages.
@@ -531,18 +520,9 @@ def _enqueue(
         # Warn if an element is being changed but the user isn't running the streamlit server.
         _maybe_print_use_warning()
 
-        # Some elements have a method.__name__ != delta_type in proto.
-        # This really matters for line_chart, bar_chart & area_chart,
-        # since add_rows() relies on method.__name__ == delta_type
-        # TODO: Fix for all elements (or the cache warning above will be wrong)
-        proto_type = delta_type
-
-        if proto_type in ARROW_DELTA_TYPES_THAT_MELT_DATAFRAMES:
-            proto_type = "arrow_vega_lite_chart"
-
         # Copy the marshalled proto into the overall msg proto
         msg = ForwardMsg_pb2.ForwardMsg()
-        msg_el_proto = getattr(msg.delta.new_element, proto_type)
+        msg_el_proto = getattr(msg.delta.new_element, delta_type)
         msg_el_proto.CopyFrom(element_proto)
 
         # Only enqueue message and fill in metadata if there's a container.
@@ -732,22 +712,20 @@ def _arrow_add_rows(
 
         # When doing _arrow_add_rows on an element that does not already have data
         # (for example, st.line_chart() without any args), call the original
-        # st._arrow_foo() element with new data instead of doing a _arrow_add_rows().
+        # st.foo() element with new data instead of doing a _arrow_add_rows().
         if (
-            self._cursor.props["delta_type"] in ARROW_DELTA_TYPES_THAT_MELT_DATAFRAMES
+            "add_rows_metadata" in self._cursor.props
+            and self._cursor.props["add_rows_metadata"]
             and self._cursor.props["add_rows_metadata"].last_index is None
         ):
-            # IMPORTANT: This assumes delta types and st method names always
-            # match!
-            # delta_type starts with "arrow_", but st_method_name doesn't use this prefix.
-            st_method_name = self._cursor.props["delta_type"].replace("arrow_", "")
-            st_method = getattr(self, st_method_name)
+            st_method = getattr(
+                self, self._cursor.props["add_rows_metadata"].chart_command
+            )
             st_method(data, **kwargs)
             return None
 
         new_data, self._cursor.props["add_rows_metadata"] = _prep_data_for_add_rows(
             data,
-            self._cursor.props["delta_type"],
             self._cursor.props["add_rows_metadata"],
         )
 
@@ -797,53 +775,21 @@ def get_last_dg_added_to_context_stack() -> DeltaGenerator | None:
 
 def _prep_data_for_add_rows(
     data: Data,
-    delta_type: str,
-    add_rows_metadata: AddRowsMetadata,
-) -> tuple[Data, AddRowsMetadata]:
-    out_data: Data
-
-    # For some delta types we have to reshape the data structure
-    # otherwise the input data and the actual data used
-    # by vega_lite will be different, and it will throw an error.
-    if delta_type in ARROW_DELTA_TYPES_THAT_MELT_DATAFRAMES:
-        import pandas as pd
-
-        df = cast(pd.DataFrame, type_util.convert_anything_to_df(data))
-
-        # Make range indices start at last_index.
-        if isinstance(df.index, pd.RangeIndex):
-            old_step = _get_pandas_index_attr(df, "step")
-
-            # We have to drop the predefined index
-            df = df.reset_index(drop=True)
-
-            old_stop = _get_pandas_index_attr(df, "stop")
-
-            if old_step is None or old_stop is None:
-                raise StreamlitAPIException(
-                    "'RangeIndex' object has no attribute 'step'"
-                )
-
-            start = add_rows_metadata.last_index + old_step
-            stop = add_rows_metadata.last_index + old_step + old_stop
-
-            df.index = pd.RangeIndex(start=start, stop=stop, step=old_step)
-            add_rows_metadata.last_index = stop - 1
-
-        out_data, *_ = prep_data(df, **add_rows_metadata.columns)
-
-    else:
+    add_rows_metadata: AddRowsMetadata | None,
+) -> tuple[Data, AddRowsMetadata | None]:
+    if not add_rows_metadata:
         # When calling add_rows on st.table or st.dataframe we want styles to pass through.
-        out_data = type_util.convert_anything_to_df(data, allow_styler=True)
+        return type_util.convert_anything_to_df(data, allow_styler=True), None
 
-    return out_data, add_rows_metadata
+    # If add_rows_metadata is set, it indicates that the add_rows used called
+    # on a chart based on our built-in chart commands.
 
+    # For built-in chart commands we have to reshape the data structure
+    # otherwise the input data and the actual data used
+    # by vega_lite will be different, and it will throw an error.
+    from streamlit.elements.lib.built_in_chart_utils import prep_chart_data_for_add_rows
 
-def _get_pandas_index_attr(
-    data: DataFrame | Series,
-    attr: str,
-) -> Any | None:
-    return getattr(data.index, attr, None)
+    return prep_chart_data_for_add_rows(data, add_rows_metadata)
 
 
 @overload