You can achieve this effect by passing RGBA tuples for the colors.

I don't know why I didn't think of this, thanks for pointing it out. Regardless it would be nice to have the alpha interface when passing in a color map and array.

One difficulty with that has to do with the underlying mechanisms of the AGG rastering. As I understand it (and I am not super familiar with that bit of code so I could be wrong), the alpha kwargs is used when the whole artist is composited into the figure and is implemented way up the class stack. Changing how this is handled could turn into a major re-write.

Just a thought but a switch could be added somewhere in the class chain that catches if alpha is an array and constructs the corresponding rgba values array from the cmap.

@acadien That does sound reasonable, I suspect the correct place is in Collection. I don't have the bandwidth to look at that right now, but if you want to that would be great!

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This is frequently asked for and we allow list alphas in other places

I'm pretty sure this works now. Adding alpha=np.linspace(0.1, 1, N) to the line collection example: https://matplotlib.org/stable/gallery/shapes_and_collections/line_collection.html shows a blended alpha.

import matplotlib.pyplot as plt
import numpy as np

from matplotlib.collections import LineCollection

N = 50
x = np.arange(N)
ys = [x + i for i in x]  # Many sets of y to plot vs. x
segs = [np.column_stack([x, y]) for y in ys]

fig, ax = plt.subplots()
ax.set_xlim(np.min(x), np.max(x))
ax.set_ylim(np.min(ys), np.max(ys))

line_segments = LineCollection(segs, array=x,
                               linewidths=(0.5, 1, 1.5, 2),
                               linestyles='solid',
                               alpha=np.linspace(0.1, 1, N))
ax.add_collection(line_segments)
axcb = fig.colorbar(line_segments)
axcb.set_label('Line Number')
ax.set_title('Line Collection with mapped colors')
plt.sci(line_segments)  # This allows interactive changing of the colormap.
plt.show()