Down arrow symbol in matplotlib

Question

I would like to create a plot where some of the points have a downward pointing arrow (see image below). In Astronomy this illustrates that the true value is actually lower than what's measured.Note that only some of the points have this symbol.

I would like to know how I can create such symbols in matplotlib. Are there downward arrow symbols that I can use?

Thanks for your help in advance!

Solution 1

Sure.

When calling matplotlibs plot function, set a marker

• If stuff like caretdown doesn't work for you, you can create your own marker by passing a list of (x,y) pairs, which are passed to a Path artist. x/y are 0…1 normalized coordinates that are scaled to the set marker size.
• You can also pass an existing Path instance as a marker, which allows even more flexibility.
• As mentioned by tom10 in the comments you can also pass a string in the form of $…$ as a marker, where … is arbitrary text, including Unicode characters, if your font supports it (should be the case these days). Downwards arrow in Unicode: ↓ (or \u2193, resulting in $\u2193$ as the marker. Note that this must be a unicode string in Python 2.x [prepend u]). Unicode Block Arrows @ Wikipedia
• You could also try passing a Arrow instance as marker, but I'm not sure whether that works.

Solution 2

The answer to my question was answered by Tom0 and Dom0. However, I just want to help the newbies like me understand how to plot those arrows. Below is the code that I found and edited to include what is said in the above example and suggestion. I hope this will help people quickly understand. I am not seeking any points.

If you like the example, please thank Dom0 and not me. :-)

import numpy as np
import matplotlib.pyplot as plt

symbols = [u'\u2193']     # this helps you get the symbol 

x = np.arange(10.)
y = np.exp(-x/2.)

plt.figure()
for i, symbol in enumerate(symbols):
    y2 = np.exp(-x/2.)
    plt.plot(x, y, 'o')              # plotting with field circles
    plt.plot(x, y2, 'g')             # plotting with green line 
    for x0, y0 in zip(x, y2):
        plt.text(x0, y0, symbol, fontname='STIXGeneral', size=30, va='center', ha='center', clip_on=True)

plt.show()

Solution 3

Take a look at this example code: http://matplotlib.org/examples/pylab_examples/errorbar_limits.html

OR: Another easy way to do this:

arrow = u'$\u2193$'
ax.plot(x, y, linestyle='none', marker=arrow, markersize=10)

Solution 4

Matplotlib supports a subset of Latex in a built-in module called mathtext. The main purpose is to properly render mathematical and scientific equations and symbols, but there's also large number of arrow symbols that can easily be used in your graphs. The following link is to the tutorial for writing math expressions. Near the bottom of the page is a list of about 80 arrow symbols.

https://matplotlib.org/users/mathtext.html#mathtext-tutorial

Examples:

plt.plot(x, y, marker=r'$\uparrow$')
plt.plot(x, y, marker=r'$\downarrow$')

View more solutions

Comments

[tom10]

+1: you can also use symbols from a font ( stackoverflow.com/questions/15866862/matplotlib-custom-marke‌​rs ), and there are plenty of arrows, but since arrows are so easy, I like the Path suggestion.

[Rohit]

Yes, \u2193 is the easiest and quickest way to get it to work!!

[tacaswell]

you can edit your question and include this code there. You can also mark your answer as community wiki (which means you won't get rep).

[Rohit]

Even for one symbol do I need to have a for loop? How would you do without the loop?

[tacaswell]

I don't understand your question. just symbol = u'\u2193' The for-loop is to make it more general.

[tom10]

@aging_gorrila: yes, I think you need the loop (but I wrote the code you copied so another opinion might help). The reason is that you're using text to plot each point and not plot, and they take different types of arguments. This is why I preferred the Path solution, since it makes a symbol that plot can use directly (though what you have works fine too). Note also, with the arrow symbol you chose, the tip of the arrow is not centered on the point.. not sure is that makes a difference to you. Finally, if this is the solution that works for you, you should accept it and not dom0's, imho.

[Rohit]

Yes, I am not quite happy that the arrow is off centered. But I thought that was the best one could get. I looked at your solution regarding path but it seemed complicated. Furthermore as a newbie, I could not find an example. Hence, I used the unicode in a loop. I would love to give you too the rep but I do not know how to give you both!

[Hermione]

Unicode chart arrows (list of all arrows): en.wikipedia.org/wiki/Template:Unicode_chart_Arrows