Custom legend of several lines with two markers for the same text

Question

I would like perform the type of legend shown in the figure

.

That figure I did with some tricks just to represent what I really want so it does not show well the color of the pink markers in the legend.

In the figure the circles represent the same parameter but for two different models; the same is true for the triangular and square markers. I want to place the two circular markers referring to the same parameter in the first line of the legend, and analogously for the two other markers on the lines below this one. Thank you.

Code:

import matplotlib.pyplot as plt

q1 = [100.0, 60.0, 200.0, 300.0]
NO1 = [0.35799999999999998, 0.33100000000000002, 0.22900000000000001,     0.17799999999999999]
No1 = [0.34599999999999997, 0.29899999999999999, 0.20699999999999999, 0.14999999999999999]
Nb1 = [0.46600000000000003, 0.45600000000000002, 0.27800000000000002, 0.24399999999999999]

q2 = [60.0, 100.0, 200.0, 300.0]
NO2 = [0.44700000000000001, 0.29199999999999998, 0.28299999999999997, 0.253]
No2 = [0.38900000000000001, 0.28499999999999998, 0.311, 0.251]
Nb2 = [0.44, 0.34899999999999998, 0.45900000000000002, 0.39400000000000002]

fig, ax = plt.subplots(figsize = (6,3))

ax.plot(q1, NO1, marker = 'o', markerfacecolor = 'none', markeredgewidth = 1, color = 'gray', linestyle = '', markersize = 8, label = '$N$ in parameter a')
ax.plot(q2, NO2, marker = 'o', markerfacecolor = 'none', markeredgewidth = 1, color = 'palevioletred', linestyle = '', markersize = 8)#, label = '$N$ in parameter a')
ax.plot(q1, No1, marker = '^', markerfacecolor = 'none', markeredgewidth = 1, color = 'gray', linestyle = '', markersize = 8, label = '$N$ in parameter b')
ax.plot(q2, No2, marker = '^', markerfacecolor = 'none', markeredgewidth = 1, color = 'palevioletred', linestyle = '', markersize = 8)#, label = '$N$ in parameter b')  
ax.plot(q1, Nb1, marker = 's', markerfacecolor = 'none', markeredgewidth = 1, color = 'gray', linestyle = '', markersize = 8, label = '$N$ in parameter c') 
ax.plot(q2, Nb2, marker = 's', markerfacecolor = 'none', markeredgewidth = 1, color = 'palevioletred', linestyle = '', markersize = 8)#, label = '$N$ in parameter c')
#plt.legend(loc='upper right', bbox_to_anchor=(0.945, 1))
plt.xlabel('x')
plt.ylabel('$N$')
plt.xticks([60, 100, 200, 300])
plt.minorticks_on()
plt.tick_params(direction = 'in', bottom = True, top = True, left = True, right = True, which = 'major')    
plt.tick_params(direction = 'in', bottom = False, top = False, left = True, right = True, which = 'minor')  

Answer 1

You may use a HandlerTuple handler and provide tuples of the artists to show in each row as handles to the legend.

import matplotlib.pyplot as plt
import matplotlib.legend_handler

q1 = [100.0, 60.0, 200.0, 300.0]
NO1 = [0.358, 0.331, 0.229, 0.178]
No1 = [0.346, 0.299, 0.207, 0.15]
Nb1 = [0.466, 0.456, 0.278, 0.244]

q2 = [60.0, 100.0, 200.0, 300.0]
NO2 = [0.447, 0.292, 0.283, 0.253]
No2 = [0.389, 0.285, 0.311, 0.251]
Nb2 = [0.44, 0.349, 0.459, 0.394]

fig, ax = plt.subplots(figsize = (6,3))

prop = dict(markerfacecolor = 'none', markeredgewidth = 1,
            linestyle = '', markersize = 8,)
l1, = ax.plot(q1, NO1, marker = 'o', color = 'gray', label = '$N$ in parameter a', **prop)
l2, = ax.plot(q2, NO2, marker = 'o', color = 'palevioletred', **prop)
l3, = ax.plot(q1, No1, marker = '^', color = 'gray', label = '$N$ in parameter b', **prop)
l4, = ax.plot(q2, No2, marker = '^', color = 'palevioletred', **prop)
l5, = ax.plot(q1, Nb1, marker = 's', color = 'gray', label = '$N$ in parameter c', **prop) 
l6, = ax.plot(q2, Nb2, marker = 's', color = 'palevioletred', **prop)


handles = [(l1,l2), (l3,l4), (l5,l6)]
_, labels = ax.get_legend_handles_labels()

ax.legend(handles = handles, labels=labels, loc='upper right', 
          handler_map = {tuple: matplotlib.legend_handler.HandlerTuple(None)})

plt.xlabel('x')
plt.ylabel('$N$')

plt.show()