How to add a legend entry without a symbol/color into an existing legend?

Question

I have a plot with a legend. I would like to add an entry into the legend box. This entry could be something like a fit parameter or something else descriptive of the data.

As an example, one can use the code below

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 2*np.pi, 101)
y = np.sin(x)

plt.plot(x, y, color='r', label='y = sin(x)')
plt.plot(np.NaN, np.NaN, color='k', label='extra label')
plt.plot(np.NaN, np.NaN, color='b', linestyle=None, label='linestyle = None')
plt.plot(np.NaN, np.NaN, color='orange', marker=None, label='marker = None')
plt.plot(np.NaN, np.NaN, color=None, label='color = None')
plt.legend()
plt.show()

to generate the plot below

I would like instead to have a label "extra label" with only whitespace and no symbol. I tried changing the linestyle, marker, and color kwargs to None but without success. I've also tried plotting plt.plot([], []) instead of plotting plt.plot(np.NaN, np.NaN). I suppose some hacky workaround is to change color='k' to color='white'. But I'm hoping there is a more proper way to do this. How can I do this?

EDIT

My question is not a duplicate. The post that this is accused of being a duplicate of shows another way of producing a legend label, but not for one without a symbol. One can run the code below to test as the same problem from my original question applies.

import matplotlib.patches as mpatches
nan_patch = mpatches.Patch(color=None, label='The label for no data')

and modify this instance from plt.legend()

plt.legend(handles=[nan_patch])

Answer 1

You can add items to legend as shown in the (now removed) duplicate. Note that to have no color in the legend itself you must set color="none", e.g

empty_patch = mpatches.Patch(color='none', label='Extra label') 
plt.legend(handles=[empty_patch])

In order to have this, as well as your existing legend entries, you can get a list of the existing legend handles and labels, add the extra ones to it, then plot the legend:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.patches as mpatches

x = np.linspace(0, 2*np.pi, 101)
y = np.sin(x)
plt.plot(x, y, color='r', label='y = sin(x)')

handles, labels = plt.gca().get_legend_handles_labels() # get existing handles and labels
empty_patch = mpatches.Patch(color='none', label='Extra label') # create a patch with no color

handles.append(empty_patch)  # add new patches and labels to list
labels.append("Extra label")

plt.legend(handles, labels) # apply new handles and labels to plot

plt.show()

Which gives:

Answer 2

A rather easy way would be to put alpha=0 in your plot function. This solution has the advantage, that you can put the text anywhere you want in the legend but not only as the last entry.

Example:

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 2*np.pi, 101)
y = np.sin(x)
plt.plot(x, y, label='y = sin(x)')
plt.plot([], [], alpha=0, label=' ') # empty line for entry separation 
plt.plot(x, -y, color='C1', label='y = -sin(x)') # because the default secondary color is skipped reassign it with color='C1'
plt.plot([], [], alpha=0, label='your label here')

plt.legend()

plt.show()