import numpy as np
import matplotlib.pyplot as plt

# make some data
x = np.linspace(0, 2*np.pi)
y1 = np.sin(x)
y2 = np.cos(x)

# plot sin(x) and cos(x)
p1 = plt.plot(x, y1, 'b-', linewidth=1.0)
p2 = plt.plot(x, y2, 'r-', linewidth=1.0)

# make a legend for both plots
leg = plt.legend([p1, p2], ['sin(x)', 'cos(x)'], loc=1)

# set the linewidth of each legend object
for legobj in leg.legendHandles:
    legobj.set_linewidth(2.0)

plt.show()

import matplotlib.pyplot as plt
from matplotlib import legend_handler
from matplotlib.lines import Line2D
import numpy as np

class MyHandlerLine2D(legend_handler.HandlerLine2D):
    def create_artists(self, legend, orig_handle,
                       xdescent, ydescent, width, height, fontsize,
                       trans):

        xdata, xdata_marker = self.get_xdata(legend, xdescent, ydescent,
                                             width, height, fontsize)

        ydata = ((height-ydescent)/2.)*np.ones(xdata.shape, float)
        legline = Line2D(xdata, ydata)

        self.update_prop(legline, orig_handle, legend)
        #legline.update_from(orig_handle)
        #legend._set_artist_props(legline) # after update
        #legline.set_clip_box(None)
        #legline.set_clip_path(None)
        legline.set_drawstyle('default')
        legline.set_marker("")
        legline.set_linewidth(10)


        legline_marker = Line2D(xdata_marker, ydata[:len(xdata_marker)])
        self.update_prop(legline_marker, orig_handle, legend)
        #legline_marker.update_from(orig_handle)
        #legend._set_artist_props(legline_marker)
        #legline_marker.set_clip_box(None)
        #legline_marker.set_clip_path(None)
        legline_marker.set_linestyle('None')
        if legend.markerscale != 1:
            newsz = legline_marker.get_markersize()*legend.markerscale
            legline_marker.set_markersize(newsz)
        # we don't want to add this to the return list because
        # the texts and handles are assumed to be in one-to-one
        # correpondence.
        legline._legmarker = legline_marker

        return [legline, legline_marker]


plt.plot( [0, 1], [0, 1], '-r', lw=1, label='Line' )
plt.legend(handler_map={Line2D:MyHandlerLine2D()})

plt.show()

import numpy as np
import matplotlib.pyplot as plt

# make some data
x = np.linspace(0, 2*np.pi)
y1 = np.sin(x)
y2 = np.cos(x)

fig, ax = plt.subplots()
ax.plot(x, y1, linewidth=1.0, label='sin(x)')
ax.plot(x, y2, linewidth=1.0, label='cos(x)')
leg = ax.legend()

for line in leg.get_lines():
    line.set_linewidth(4.0)

plt.show()