matplotlib axis arrow tip

Question

I am trying to set an arrow at the end of a an axis in matplotlib. I don't want to remove the spines and replace them with pure arrows because I need their functionalities ...

my implementation is as slight modification of joferkington implementation

import matplotlib.pyplot as plt
import numpy as np

def arrowed_spines(ax=None, arrowLength=30, labels=('X', 'Y'), arrowStyle='<|-'):
    xlabel, ylabel = labels

    for i, spine in enumerate(['left', 'bottom']):
        # Set up the annotation parameters
        t = ax.spines[spine].get_transform()
        xy, xycoords = [1, 0], ('axes fraction', t)
        xytext, textcoords = [arrowLength, 0], ('offset points', t)

        # create arrowprops
        arrowprops = dict( arrowstyle=arrowStyle,
                           facecolor=ax.spines[spine].get_facecolor(), 
                           linewidth=ax.spines[spine].get_linewidth(),
                           alpha = ax.spines[spine].get_alpha(),
                           zorder=ax.spines[spine].get_zorder(),
                           linestyle = ax.spines[spine].get_linestyle() )

        if spine is 'bottom':
            ha, va = 'left', 'center'
            xarrow = ax.annotate(xlabel, xy, xycoords=xycoords, xytext=xytext, 
                        textcoords=textcoords, ha=ha, va='center',
                        arrowprops=arrowprops)
        else:
            ha, va = 'center', 'bottom'
            yarrow = ax.annotate(ylabel, xy[::-1], xycoords=xycoords[::-1], 
                        xytext=xytext[::-1], textcoords=textcoords[::-1], 
                        ha='center', va=va, arrowprops=arrowprops)
    return xarrow, yarrow

# plot
x = np.arange(-2., 10.0, 0.01)
plt.plot(x, x**2)
plt.gcf().set_facecolor('white')    
ax = plt.gca()
ax.set_xticks([]) 
ax.set_yticks([]) 

ax.spines['left'].set_position('center')
ax.spines['right'].set_color('none')
ax.spines['bottom'].set_position('center')
ax.spines['top'].set_color('none')

arrowed_spines(ax)

plt.show()

The plot result shows a shift in the arrow as the following

It seems that a shift of a point or two is consistent in the starting position and the alignment of the arrow with the spine. I don't know how to fix this problem. Any help would be appreciated.

thanks

Answer 1

There is an example showing how to get arrows as axis decorators in the matplotlib documentation using the mpl_toolkits.axisartist toolkit:

from mpl_toolkits.axisartist.axislines import SubplotZero
import matplotlib.pyplot as plt
import numpy as np


fig = plt.figure()
ax = SubplotZero(fig, 111)
fig.add_subplot(ax)

for direction in ["xzero", "yzero"]:
    # adds arrows at the ends of each axis
    ax.axis[direction].set_axisline_style("-|>")

    # adds X and Y-axis from the origin
    ax.axis[direction].set_visible(True)

for direction in ["left", "right", "bottom", "top"]:
    # hides borders
    ax.axis[direction].set_visible(False)

x = np.linspace(-0.5, 1., 100)
ax.plot(x, np.sin(x*np.pi))

plt.show()

---

For many cases, the use of the mpl_toolkits.axisartist.axislines module is not desired. In that case one can also easily get arrow heads by using triangles as markers on the top of the spines:

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(-np.pi, np.pi, 100)
y = 2 * np.sin(x)

rc = {"xtick.direction" : "inout", "ytick.direction" : "inout",
      "xtick.major.size" : 5, "ytick.major.size" : 5,}
with plt.rc_context(rc):
    fig, ax = plt.subplots()
    ax.plot(x, y)

    ax.spines['left'].set_position('zero')
    ax.spines['right'].set_visible(False)
    ax.spines['bottom'].set_position('zero')
    ax.spines['top'].set_visible(False)
    ax.xaxis.set_ticks_position('bottom')
    ax.yaxis.set_ticks_position('left')

    # make arrows
    ax.plot((1), (0), ls="", marker=">", ms=10, color="k",
            transform=ax.get_yaxis_transform(), clip_on=False)
    ax.plot((0), (1), ls="", marker="^", ms=10, color="k",
            transform=ax.get_xaxis_transform(), clip_on=False)

    plt.show()