matplotlib fill_between facecolor alpha vs edgecolor alpha?

Question

Is there a way to either specify different alphas for facecolor vs edgecolor? Or is there a way to plot an alpha filled area with non-alpha edgecolor that also works in the legend?

This is not what I want...

axs.fill_between(xvalues, tupper_w, tlower_w, facecolor='dimgray', edgecolor='dimgray', alpha=0.25, label='$measured\quad\sigma$')
axs.fill_between(xvalues, pupper_w, plower_w, facecolor='orange', edgecolor='orange', alpha=0.25, label='$predicted\quad\sigma$')
axs.plot(xvalues, tcurvesavg_w, color='dimgray', label='$\overline{measured}$', ls='--')
axs.plot(xvalues, pcurvesavg_w, color='orange', label='$\overline{predicted}$', ls='--')

This is what I want (but with proper legend):

axs.fill_between(xvalues, tupper, tlower, facecolor='dimgray', alpha=0.25, label='$measured\quad\sigma$')
axs.fill_between(xvalues, pupper, plower, facecolor='orange', alpha=0.25, label='$predicted\quad\sigma$')
axs.plot(xvalues, tupper, color='dimgray', lw=0.5)
axs.plot(xvalues, tcurvesavg, color='dimgray', label='$\overline{measured}$', ls='--')
axs.plot(xvalues, tlower, color='dimgray', lw=0.5)
axs.plot(xvalues, pupper, color='orange', lw=0.5)
axs.plot(xvalues, pcurvesavg, color='orange', label='$\overline{predicted}$', ls='--')
axs.plot(xvalues, plower, color='orange', lw=0.5)

Answer 1

You cannot specify different alpha values via the alpha argument. However you can define each of facecolor and edgecolor with an alpha channel, e.g. for red with 40% opacity

facecolor=(1,0,0,.4)

This is then directly applied in the legend.

import matplotlib.pyplot as plt
import numpy as np

x = np.arange(0.0, 2, 0.01)
y1 = np.sin(2*np.pi*x)
y2 = 1.2*np.sin(4*np.pi*x)-.9

fig, ax = plt.subplots()

ax.fill_between(x, y1, y1+.5, facecolor=(1,0,0,.4), edgecolor=(0,0,0,.5), label="Label 1")
ax.fill_between(x, y2, y2-.5, facecolor=(0,0,1,.4), edgecolor=(0,0,0,.5), label="Label 1")

ax.legend()
plt.show()

Answer 2

Immediately, looking at fill_between alpha and general fill_between documentation it appears to be unsupported. The legend documentation doesn't seem to provide an option for adding your border after plotting either.

In your second code snippet, if you can figure out how to get the plot and fill functions to have a single handle then the legend should automatically format. Something similar to below (adapted from this similar, but not quite duplicate StackExchangePost):

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

xvalues = np.linspace(0,1,11)
tcurvesavg = np.linspace(0,1,11)
p1, = plt.plot(xvalues, tcurvesavg , c='r')  # notice the comma!
p2 = plt.fill_between(xvalues, tcurvesavg -0.2, tcurvesavg +0.2, color='r', alpha=0.5)
plt.legend(((p1,p2),), ('Entry',))
plt.show()

(As a non-automated workaround for most matplotlib questions, save as a svg (similar to this post) and add a border in a vector graphics program like Inkscape. You shouldn't lose resolution, and could still put it in reports etc.)