Using matplotlib and point to specific value on the x-axis

Question

I need to "highlight" a few positions on the x-axis of my charts/graphs where significant events occurred. Since the chart/graph is really all over the place I don't want to add vlines inside the chart/graph area itself but rather add arrows, or lines, on top of the chart/graph area together with a number I can refer to later on when mentioning the event in writing ((A) for example).

I've managed to plot vlines, but the label isn't visible at all. Most of all I would like arrows instead of lines since that would be more clear to the reader...

How I'm plotting right now

plt.vlines(
    x = position[0], 
    ymin = axis_ymax, 
    ymax = axis_ymax + int(axis_ymax * 0.05), 
    linestyles = 'solid', 
    label = '(A)'
).set_clip_on(False)

Ffisegydd · Accepted Answer · 2014-04-26 13:51:04Z

9

You can use ax.annotate to annotate a point in your graph with an arrow.

Using ax.annotate you can choose the annotation, the coordinates of the arrow head (xy), the coordinates of the text (xytext), and any properties you want the arrow to have (arrowprops).

import numpy as np

import matplotlib.pyplot as plt

# Generate some data
x = np.linspace(0, 10, 1000)
y = np.sin(np.exp(0.3*x))

fig, ax = plt.subplots()

ax.plot(x, y)

ax.set_ylim(-2,2)

ax.annotate('First maxima', 
            xy=(np.pi/2., 2), 
            xytext=(np.pi/2., 2.3), 
            arrowprops = dict(facecolor='black', shrink=0.05))

plt.show()

Plot

edited Apr 26, 2014 at 13:51

answered Apr 26, 2014 at 13:27

Ffisegydd

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2 Comments

user3235200 Over a year ago

Can the arrow be located entirely outside of the graph area? i.e pointing to the x-axis from below, or pointing towards the x-axis from above the "ceiling", so to speak?

Ffisegydd Over a year ago

Yes if you were to use ax.annotate('First maxima', xy=(np.pi/2., 2), xytext=(np.pi/2., 2.3), arrowprops = dict(facecolor='black', shrink=0.05)) then it would place the arrow at the top of your plot with the text above it. Be careful that you don't place the text too far above the plot though or it will go off screen.

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