Polar plot - Put one grid line in bold

Question

I am trying to make use the polar plot projection to make a radar chart. I would like to know how to put only one grid line in bold (while the others should remain standard).

For my specific case, I would like to highlight the gridline associated to the ytick "0".

from matplotlib import pyplot as plt
import pandas as pd
import numpy as np
#Variables
sespi = pd.read_csv("country_progress.csv")
labels = sespi.country
progress = sespi.progress
angles=np.linspace(0, 2*np.pi, len(labels), endpoint=False)
#Concatenation to close the plots
progress=np.concatenate((progress,[progress[0]]))
angles=np.concatenate((angles,[angles[0]]))

#Polar plot
fig=plt.figure()
ax = fig.add_subplot(111, polar=True)
ax.plot(angles, progress, '.--', linewidth=1, c="g")
#ax.fill(angles, progress, alpha=0.25)
ax.set_thetagrids(angles * 180/np.pi, labels)
ax.set_yticklabels([-200,-150,-100,-50,0,50,100,150,200])
#ax.set_title()
ax.grid(True)
plt.show()

Answer 1

The gridlines of a plot are Line2D objects. Therefore you can't make it bold. What you can do (as shown, in part, in the other answer) is to increase the linewidth and change the colour but rather than plot a new line you can do this to the specified gridline.

You first need to find the index of the y tick labels which you want to change:

y_tick_labels = [-100,-10,0,10]
ind = y_tick_labels.index(0) # find index of value 0

You can then get a list of the gridlines using gridlines = ax.yaxis.get_gridlines(). Then use the index you found previously on this list to change the properties of the correct gridline.

Using the example from the gallery as a basis, a full example is shown below:

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

ax = plt.subplot(111, projection='polar')
ax.set_rmax(2)
ax.set_rticks([0.5, 1, 1.5, 2])  # less radial ticks
ax.set_rlabel_position(-22.5)  # get radial labels away from plotted line
ax.grid(True)

y_tick_labels = [-100, -10, 0, 10]
ax.set_yticklabels(y_tick_labels)
ind = y_tick_labels.index(0)  # find index of value 0

gridlines = ax.yaxis.get_gridlines()
gridlines[ind].set_color("k")
gridlines[ind].set_linewidth(2.5)

plt.show()

Which gives: