Add get_axis_by_index function

src/corner/__init__.py

@@ -1,7 +1,22 @@
 # -*- coding: utf-8 -*-
 
-__all__ = ["corner", "hist2d", "quantile", "overplot_lines", "overplot_points"]
+__all__ = [
+    "corner",
+    "hist2d",
+    "quantile",
+    "overplot_lines",
+    "overplot_points",
+    "axis_from_param_indices",
+    "param_indices_from_axis",
+]
 
-from corner.core import hist2d, overplot_lines, overplot_points, quantile
+from corner.core import (
+    axis_from_param_indices,
+    hist2d,
+    overplot_lines,
+    overplot_points,
+    param_indices_from_axis,
+    quantile,
+)
 from corner.corner import corner
 from corner.version import version as __version__

src/corner/core.py

@@ -892,6 +892,84 @@ def overplot_points(fig, xs, reverse=False, **kwargs):
                 axes[k2, k1].plot(xs[k1], xs[k2], **kwargs)
 
 
+def axis_from_param_indices(fig, ix, iy, return_axis=True):
+    """
+    Get axis corresponding to indices ``ix``, ``iy`` of the input data.  This can be used, e.g., for
+    manually adding additional data or labels to a specific axis.  This is the inverse of
+    `param_indices_from_axis`.
+
+    Parameters
+    ----------
+    fig : Figure
+        The figure generated by a call to :func:`corner.corner`.
+
+    ix, iy : int
+        Indices of the parameter list corresponding to the plotted ``x`` and ``y`` axes.  Only cases
+        where ``ix <= iy`` have plotted axes, and ``ix == iy`` corresponds to the histogram axis for
+        parameter index ``ix``.  The function doesn't raise an error when ``ix > iy`` corresponding to one
+        of the hidden axes, though it does raise an error if either ``ix`` or ``iy`` is too large for the
+        dimensions of the plotted ``fig``.
+
+    return_axis : bool
+        Return either the axis itself or its integer index
+
+    Returns
+    -------
+    ax : axis
+        Entry in the ``fig.axes`` list.
+    """
+    ndim = int(np.sqrt(len(fig.axes)))
+    if ix > ndim - 1:
+        msg = f"ix={ix} too large for ndim={ndim}"
+        raise ValueError(msg)
+    elif iy > ndim - 1:
+        msg = f"ix={ix} too large for ndim={ndim}"
+        raise ValueError(msg)
+
+    for i in range(ndim**2):
+        ix_i = range(ndim)[(i % ndim)]
+        iy_i = range(ndim)[(i // ndim) - ndim]
+
+        if (ix == ix_i) & (iy == iy_i):
+            break
+
+    if return_axis:
+        return fig.axes[i]
+    else:
+        return i
+
+
+def param_indices_from_axis(fig, i):
+    """
+    Get indices ``ix``, ``iy`` of the input data associated with one of the plotted axes.  This is the
+    inverse of `axis_from_param_indices`.
+
+    Parameters
+    ----------
+    fig : Figure
+        The figure generated by a call to :func:`corner.corner`.
+
+    i : int
+        Index of an entry in the ``fig.axes`` list
+
+    Returns
+    -------
+    ix, iy : int
+        Indices of the figure axes list corresponding to the plotted ``x`` and ``y`` of the specified axis
+        index
+    """
+    if i > len(fig.axes):
+        msg = f"{i} must be < len(fig.axes) = {len(fig.axes)}"
+        raise ValueError(msg)
+
+    ndim = int(np.sqrt(len(fig.axes)))
+
+    ix = range(ndim)[(i % ndim)]
+    iy = range(ndim)[(i // ndim) - ndim]
+
+    return ix, iy
+
+
 def _parse_input(xs):
     xs = np.atleast_1d(xs)
     if len(xs.shape) == 1:

tests/test_corner.py

@@ -65,6 +65,46 @@ def test_basic():
     _run_corner()
 
 
+def test_axis_index():
+
+    labels = ["a", "b", "c"]
+    fig = _run_corner(labels=labels, n=100)
+
+    # This should be x=a vs. y=c plotted in the lower left corner with both labels
+    ax = corner.axis_from_param_indices(fig, 0, 2)
+    assert ax.get_xlabel() == labels[0]
+    assert ax.get_ylabel() == labels[2]
+
+    # This should be x=b vs. y=c, to the right of the previous with no y label
+    ax = corner.axis_from_param_indices(fig, 1, 2)
+    assert ax.get_xlabel() == labels[1]
+    assert ax.get_ylabel() == ""
+
+    # This should be the histogram of c at the lower right
+    ax = corner.axis_from_param_indices(fig, 2, 2)
+
+    # Some big number, probably 1584 depending on the seed?
+    assert ax.get_ylim()[1] > 100
+
+    # ix > iy is hidden, which have ranges set to (0,1)
+    ax = corner.axis_from_param_indices(fig, 2, 1)
+    assert np.allclose(ax.get_xlim(), [0, 1])
+    assert np.allclose(ax.get_ylim(), [0, 1])
+
+    with pytest.raises(ValueError):
+        ax = corner.axis_from_param_indices(fig, 2, 4)
+
+    # Inverse
+    for ix in range(len(labels)):
+        for iy in range(ix + 1, len(labels)):
+            i = corner.axis_from_param_indices(fig, ix, iy, return_axis=False)
+            ix_i, iy_i = corner.param_indices_from_axis(fig, i)
+            assert np.allclose([ix_i, iy_i], [ix, iy])
+
+    with pytest.raises(ValueError):
+        _ = corner.param_indices_from_axis(fig, 100)
+
+
 @image_comparison(
     baseline_images=["basic_log"], remove_text=True, extensions=["png"]
 )