Add the Position class for GMT embellishment placement

doc/api/index.rst

@@ -214,6 +214,7 @@ Class-style Parameters
 
     Box
     Pattern
+    Position
 
 Enums
 -----

pygmt/params/__init__.py

@@ -4,3 +4,4 @@
 
 from pygmt.params.box import Box
 from pygmt.params.pattern import Pattern
+from pygmt.params.position import Position

pygmt/params/position.py

@@ -0,0 +1,209 @@
+"""
+The Position class for positioning GMT embellishments.
+"""
+
+import dataclasses
+from collections.abc import Sequence
+from typing import Literal
+
+from pygmt._typing import AnchorCode
+from pygmt.alias import Alias
+from pygmt.exceptions import GMTValueError
+from pygmt.params.base import BaseParam
+
+
+@dataclasses.dataclass(repr=False)
+class Position(BaseParam):
+    """
+    Class for positioning embellishments on a plot.
+
+    .. figure:: https://docs.generic-mapping-tools.org/dev/_images/GMT_anchor.png
+       :width: 600 px
+       :align: center
+
+       The placement of a GMT embellishment (represented by a green rectangle) in
+       relation to the underlying plot (represented by a bisque rectangle).
+
+    This class provides flexible positioning for GMT embellishments (e.g., logo, scale,
+    rose) by defining a *reference point* on the plot and an *anchor point* on the
+    embellishment. The embellishment is positioned so these two points overlap.
+
+    **Conceptual Model**
+
+    Think of it like dropping an anchor from a boat:
+
+    1. The boat navigates to the *reference point* (a location on the plot)
+    2. The *anchor point* (a specific point on the embellishment) is aligned with the
+       *reference point*
+    3. The embellishment is "dropped" at that position
+
+    **Reference Point**
+
+    The *reference point* can be specified in five different ways using the ``type`` and
+    ``location`` attributes:
+
+    ``type="mapcoords"`` Map Coordinates
+        Use data/geographic coordinates. Specify ``location`` as
+        (*longitude*, *latitude*). Useful when tying the embellishment to a specific
+        geographic location.
+
+        **Example:** ``location=(135, 20), type="mapcoords"``.
+
+    ``type="plotcoords"`` Plot Coordinates
+        Use plot coordinates as distances from the lower-left plot origin. Specify
+        ``location`` as (*x*, *y*) with units (e.g., inches, centimeters, points).
+        Useful for precise layout control.
+
+        **Example:** ``location=("2c", "2.5c"), type="plotcoords"``
+
+    ``type="boxcoords"`` Normalized Coordinates
+        Use normalized coordinates where (0, 0) is the lower-left corner and (1, 1) is
+        the upper-right corner of the bounding box of the current plot. Specify
+        ``location`` as (*nx*, *ny*). Useful for positioning relative to plot dimensions
+        without units.
+
+        **Example:** ``location=(0.2, 0.1), type="boxcoords"``
+
+    ``type="inside"`` Inside Plot
+        Select one of the nine :doc:`justification codes </techref/justification_codes>`
+        as the *reference point*. The *anchor point* defaults to be the same as the
+        *reference point*, so the embellishment is placed inside the plot.
+
+        **Example:** ``location="TL", type="inside"`` [anchor point defaults to "TL"]
+
+    ``type="outside"`` Outside Plot
+        Similar to ``type="inside"``, but the *anchor point* defaults to the mirror
+        opposite of the *reference point*. Useful for placing embellishments outside
+        the plot boundaries (e.g., color bars).
+
+        **Example:** ``location="TL", type="outside"`` [anchor point defaults to "BR"]
+
+    **Anchor Point**
+
+    The *anchor point* determines which part of the embellishment aligns with the
+    *reference point*. It uses one of nine
+    :doc:`justification codes </techref/justification_codes>`.
+
+    Set ``anchor`` explicitly to override these defaults. If not set, the default
+    *anchor* behaviors are:
+
+    - ``type="inside"``: Same as the *reference point* justification code
+    - ``type="outside"``: Mirror opposite of the *reference point* justification code
+    - Other types: ``"MC"`` (middle center) for map rose and scale, ``"BL"``
+      (bottom-left) for other embellishments
+
+    **Offset**
+
+    The ``offset`` parameter shifts the *anchor point* from its default position.
+    Offsets are applied to the projected plot coordinates, with positive values moving
+    in the direction indicated by the *anchor point*'s justification code. It should be
+    a single value (applied to both x and y) or as (*offset_x*, *offset_y*).
+
+    Examples
+    --------
+    Position the GMT logo at map coordinates (3, 3) with the logo's middle-left point as
+    the anchor, offset by (0.2, 0.2):
+
+    >>> import pygmt
+    >>> from pygmt.params import Position
+    >>> fig = pygmt.Figure()
+    >>> fig.basemap(region=[0, 10, 0, 10], projection="X10c", frame=True)
+    >>> fig.logo(
+    ...     position=Position((3, 3), type="mapcoords", anchor="ML", offset=(0.2, 0.2)),
+    ...     box=True,
+    ... )
+    >>> fig.show()
+
+    Position the GMT logo at the top-left corner inside the plot:
+
+    >>> fig = pygmt.Figure()
+    >>> fig.basemap(region=[0, 10, 0, 10], projection="X10c", frame=True)
+    >>> fig.logo(position=Position("TL", type="inside", offset="0.2c"), box=True)
+    >>> fig.show()
+    """
+
+    #: Location of the reference point on the plot. The format depends on ``type``:
+    #:
+    #: - ``type="mapcoords"``: (*longitude*, *latitude*)
+    #: - ``type="plotcoords"``: (*x*, *y*) with plot units
+    #: - ``type="boxcoords"``: (*nx*, *ny*)
+    #: - ``type="inside"`` or ``"outside"``:
+    #:   :doc:`2-character justification codes </techref/justification_codes>`
+    location: Sequence[float | str] | AnchorCode
+
+    #: Types of the reference point. Valid values are:
+    #:
+    #: - ``"mapcoords"``: Map/Data coordinates
+    #: - ``"plotcoords"``: Plot coordinates
+    #: - ``"boxcoords"``: Normalized coordinates
+    #: - ``"inside"`` or ``"outside"``: Justification codes
+    #:
+    #: If not specified, defaults to ``"inside"`` if ``location`` is a justification
+    #: code; otherwise defaults to ``"plotcoords"``.
+    type: (
+        Literal["mapcoords", "inside", "outside", "boxcoords", "plotcoords"] | None
+    ) = None
+
+    #: Anchor point on the embellishment using a
+    #: :doc:`2-character justification codes </techref/justification_codes>`.
+    #: If ``None``, defaults are applied based on ``type`` (see above).
+    anchor: AnchorCode | None = None
+
+    #: Offset for the anchor point as a single value or (*offset_x*, *offset_y*).
+    #: If a single value is given, the offset is applied to both x and y directions.
+    offset: Sequence[float | str] | None = None
+
+    def _validate(self):
+        """
+        Validate the parameters.
+        """
+        _valid_anchors = {f"{h}{v}" for v in "TMB" for h in "LCR"} | {
+            f"{v}{h}" for v in "TMB" for h in "LCR"
+        }
+
+        # Default to "inside" if type is not specified and location is an anchor code.
+        if self.type is None:
+            self.type = "inside" if isinstance(self.location, str) else "plotcoords"
+
+        # Validate the location based on type.
+        match self.type:
+            case "mapcoords" | "plotcoords" | "boxcoords":
+                if not isinstance(self.location, Sequence) or len(self.location) != 2:
+                    raise GMTValueError(
+                        self.location,
+                        description="reference point",
+                        reason="Expect a sequence of two values.",
+                    )
+            case "inside" | "outside":
+                if self.location not in _valid_anchors:
+                    raise GMTValueError(
+                        self.location,
+                        description="reference point",
+                        reason="Expect a valid 2-character justification code.",
+                    )
+        # Validate the anchor if specified.
+        if self.anchor is not None and self.anchor not in _valid_anchors:
+            raise GMTValueError(
+                self.anchor,
+                description="anchor point",
+                reason="Expect a valid 2-character justification code.",
+            )
+
+    @property
+    def _aliases(self):
+        return [
+            Alias(
+                self.type,
+                name="type",
+                mapping={
+                    "mapcoords": "g",
+                    "boxcoords": "n",
+                    "plotcoords": "x",
+                    "inside": "j",
+                    "outside": "J",
+                },
+            ),
+            Alias(self.location, name="location", sep="/", size=2),
+            Alias(self.anchor, name="anchor", prefix="+j"),
+            Alias(self.offset, name="offset", prefix="+o", sep="/", size=2),
+        ]

pygmt/tests/test_params_position.py

@@ -0,0 +1,58 @@
+"""
+Test the Position class.
+"""
+
+import pytest
+from pygmt.exceptions import GMTValueError
+from pygmt.params import Position
+
+
+def test_params_position_types():
+    """
+    Test the Position class with different types of coordinate systems.
+    """
+    # Default type is "plotcoords" for (x,y) and "inside" for anchor codes.
+    assert str(Position((1, 2))) == "x1/2"
+    assert str(Position("TL")) == "jTL"
+
+    assert str(Position((10, 20), type="mapcoords")) == "g10/20"
+    assert str(Position((0.1, 0.2), type="boxcoords")) == "n0.1/0.2"
+    assert str(Position(("5c", "3c"), type="plotcoords")) == "x5c/3c"
+    assert str(Position("MR", type="inside")) == "jMR"
+    assert str(Position("BR", type="outside")) == "JBR"
+
+
+def test_params_position_anchor_offset():
+    """
+    Test the Position class with anchor and offset parameters.
+    """
+    assert str(Position((10, 20), type="mapcoords", anchor="TL")) == "g10/20+jTL"
+    assert str(Position((10, 20), type="mapcoords", offset=(1, 2))) == "g10/20+o1/2"
+    pos = Position("TL", type="inside", anchor="MC", offset=("1c", "2c"))
+    assert str(pos) == "jTL+jMC+o1c/2c"
+
+
+def test_params_position_invalid_location():
+    """
+    Test that invalid location inputs raise GMTValueError.
+    """
+    with pytest.raises(GMTValueError):
+        Position("invalid", type="mapcoords")
+    with pytest.raises(GMTValueError):
+        Position((1, 2, 3), type="mapcoords")
+    with pytest.raises(GMTValueError):
+        Position(5, type="plotcoords")
+    with pytest.raises(GMTValueError):
+        Position((0.5,), type="boxcoords")
+    with pytest.raises(GMTValueError):
+        Position((10, 20), type="inside")
+    with pytest.raises(GMTValueError):
+        Position("TT", type="outside")
+
+
+def test_params_position_invalid_anchor():
+    """
+    Test that invalid anchor inputs raise GMTValueError.
+    """
+    with pytest.raises(GMTValueError):
+        Position((10, 20), type="mapcoords", anchor="XX")