python - Arrow on a line plot with matplotlib

Question

I'd like to add an arrow to a line plot with matplotlib like in the plot below (drawn with pgfplots).

How can I do (position and direction of the arrow should be parameters ideally)?

Here is some code to experiment.

from matplotlib import pyplot
import numpy as np

t = np.linspace(-2, 2, 100)
plt.plot(t, np.sin(t))
plt.show()

Thanks.

Answer 1

In my experience this works best by using annotate. Thereby you avoid the weird warping you get with ax.arrow which is somehow hard to control.

EDIT: I've wrapped it into a little function.

from matplotlib import pyplot as plt
import numpy as np


def add_arrow(line, position=None, direction='right', size=15, color=None):
    """
    add an arrow to a line.

    line:       Line2D object
    position:   x-position of the arrow. If None, mean of xdata is taken
    direction:  'left' or 'right'
    size:       size of the arrow in fontsize points
    color:      if None, line color is taken.
    """
    if color is None:
        color = line.get_color()

    xdata = line.get_xdata()
    ydata = line.get_ydata()

    if position is None:
        position = xdata.mean()
    # find closest index
    start_ind = np.argmin(np.absolute(xdata - position))
    if direction == 'right':
        end_ind = start_ind + 1
    else:
        end_ind = start_ind - 1

    line.axes.annotate('',
        xytext=(xdata[start_ind], ydata[start_ind]),
        xy=(xdata[end_ind], ydata[end_ind]),
        arrowprops=dict(arrowstyle="->", color=color),
        size=size
    )


t = np.linspace(-2, 2, 100)
y = np.sin(t)
# return the handle of the line
line = plt.plot(t, y)[0]

add_arrow(line)

plt.show()

It's not very intuitive but it works. You can then fiddle with the arrowprops dictionary until it looks right.