如何为极坐标matplotlib创建次要刻度

Question

I am interested in the following two things for the polar plot plotted by matplotlib shown below

1. How do I create minor ticks for a polar plot on the r axis?
2. How do I move the r labels further away from the r ticks, as seen in the graph, some of the r ticks are in contact with the axis.

Answer 1

The polar plot does not have minor or major ticks. So I think you need to create the minor ticks manually by plotting small line segments.

For example:

import numpy as np
import matplotlib.pyplot as plt

r = np.arange(0, 2, 0.01)
theta = 2 * np.pi * r

ax = plt.subplot(111, projection='polar')
ax.plot(theta, r)
ax.set_rmax(2)
ax.margins(y=0)
ax.set_rticks([0.5, 1, 1.5, 2])  # less radial ticks
ax.set_rlabel_position(120)  # get radial labels away from plotted line

ax.grid(True)

tick = [ax.get_rmax(),ax.get_rmax()*0.97]
for t  in np.deg2rad(np.arange(0,360,5)):
    ax.plot([t,t], tick, lw=0.72, color="k")

ax.set_title("A line plot on a polar axis", va='bottom')
plt.show()

Answer 2

For your first question, you can either increase the number of ticks, which doesn't seem to be what you want if you wish for minor ticks, or you can manually generate the ticks yourself. To do this you will need to use the polar axes own plot facilities to plot these ticks ie:

ax.plot([theta_start, theta_end], [radius_start, radius_end], kwargs**)

You'll need to figure out the interval you want these ticks, and then tick them manually with a function like the one below.

def minor_tick_gen(polar_axes, tick_depth, tick_degree_interval, **kwargs):
    for theta in np.deg2rad(range(0, 360, tick_degree_interval)):
        polar_axes.plot([theta, theta], [polar_axes.get_rmax(), polar_axes.get_rmax()-tick_depth], **kwargs)

which you can then call like this:

minor_tick_gen(ax, 0.25, 20, color = "black")

Its kind of difficult to find, but polar axis are not normal axes, but Polar Axis class instances. In the documentation you can use set_ylim(min, max) which will allow you to move the labels off of the line, however this will rescale the entire graph. Going outside of the graph bounds will require developer knowledge of the framework, because matplotlib does not expose this functionality to you. Using set_rgrids(...) for example, even with a position component will not affect the relative label positioning.

Putting these things together, you can use the following code:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.ticker as ticker
import math


def minor_tick_gen(polar_axes, tick_depth, tick_degree_interval, **kwargs):
    for theta in np.deg2rad(range(0, 360, tick_degree_interval)):
        polar_axes.plot([theta, theta], [polar_axes.get_rmax(), polar_axes.get_rmax()-tick_depth], **kwargs)

def radian_function(x, y =None):
    rad_x = x/math.pi
    return "{}π".format(str(rad_x if rad_x % 1 else int(rad_x)))

ax = plt.subplot(111, projection='polar')
ax.set_rmax(2)
ax.set_rticks([3*math.pi, 6*math.pi, 9*math.pi, 12*math.pi])
ax.set_rlabel_position(112.5)
# go slightly beyond max value for ticks to solve second problem
ax.set_ylim(0, 13*math.pi)
ax.grid(True)
# generate ticks for first problem
minor_tick_gen(ax, math.pi, 20, color = "black", lw = 0.5)
ax.set_title("Polar axis label minor tick example", va='bottom')
ax.yaxis.set_major_formatter(ticker.FuncFormatter(radian_function))
ax.xaxis.set_major_formatter(ticker.FuncFormatter(radian_function))
plt.show()

to get the following image