In a matplotlib-plot, add a label to a curve according to its displayed slope, independent of potential log-scales

add_curve_label.py

import numpy as np
from matplotlib import pyplot as pl

def add_curve_label(ax,
                    curve_x,
                    curve_y,
                    label,
                    label_pos_abs=None,
                    label_pos_rel=None,
                    bbox_pad=1.0,
                    **kwargs):
    """
    Add a label to a curve according to the curve's slope
    on the displayed figure.

    Parameters
    ----------
    ax : matplotlib.Axes
        The ax object where to put the label on. Use
        `pyplot.gca()` to get the current focal axes.
    curve_x : numpy.ndarray
        The curve's x-data.
    curve_y : numpy.ndarray
        The curve's y-data.
    label : str
        The label.
    label_pos_abs : float, default : None
        The absolute x-position at which to pose the label.
        Must be smaller than `curve_x`'s last element.
        If None, `label_pos_rel` must be given.
    label_pos_rel : float, default : None
        The relative x-position at which to pose the label.
        Must be 0 <= label_pos_rel < 1.
        If None, `label_pos_abs` must be given.
    bbox_pad : float, default : 1.0
        Padding of the bounding box around the label.
    **kwargs
        Will be passed to pyplot.text.
    """

    if label_pos_abs is None and label_pos_rel is not None:

        # get xmin and xmax in display coordinates
        xmin = ax.transData.transform(np.array( [ curve_x[1],  curve_y[1]  ] ))[0]
        xmax = ax.transData.transform(np.array( [ curve_x[-1], curve_y[-1] ] ))[0]

        # compute label x-position in display coordinates according to
        # demanded relative label position
        new_display_x = xmin + label_pos_rel * (xmax - xmin)

        # convert back to data coordinates and save absolute position
        new_data_x = ax.transData.inverted().transform(np.array([new_display_x,1.0]))
        label_pos_abs = new_data_x[0]

    elif label_pos_abs is None and label_pos_rel is None:
        raise ValueError('Please provide either `label_pos_abs` or `label_pos_rel`.')
    elif label_pos_abs is not None and label_pos_rel is not None:
        raise ValueError('Please provide either `label_pos_abs` or `label_pos_rel`, not both.')

    # find ndx in data for demanded label position
    ndx = np.where(curve_x < label_pos_abs)[0][-1]

    # convert data at this point to display coordinates
    x0, y0 = ax.transData.transform( np.array( [ curve_x[ndx], curve_y[ndx] ] ))
    x1, y1 = ax.transData.transform( np.array( [ curve_x[ndx+1], curve_y[ndx+1] ] ))

    # compute slope and angle at this point in display coordinates
    dx = x1 - x0
    dy = y1 - y0
    angle = np.arctan2(dy,dx) / np.pi * 180

    # convert back to data coordinates
    x0 = label_pos_abs
    y0 = np.interp(x0, curve_x, curve_y)
    # define bounding box for label
    bbox = dict(facecolor='w', alpha=1, edgecolor='none', pad=bbox_pad)

    if not ('ha' in kwargs or 'horizontalalignment' in kwargs):
        kwargs['ha'] = 'center'

    if not ('va' in kwargs or 'verticalalignment' in kwargs):
        kwargs['va'] = 'center'

    # add label
    ax.text(x0,
            y0,
            label,
            rotation=angle,
            rotation_mode='anchor',
            bbox=bbox,
            transform=ax.transData,
            **kwargs
            )