<｜begin▁of▁sentence｜>### Instruction:
As an endocrinologist, suppose you are visualizing a hormonal signaling pathway network, with directed signal transduction from one gland to another. How can you use the AlphaVaryingEdgeDrawer in igraph to represent the strength of hormonal signals, with alpha values varying from the originating gland to the destination gland?
### Response:
import igraph as ig
import cairo

# Create a graph with directed edges
g = ig.Graph(directed=True)
g.add_vertices(3)
g.add_edges([(0, 1), (1, 2)])

# Define the size of the image
width, height = 400, 400

# Create a Cairo Image Surface to draw on
surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, width, height)
ctx = cairo.Context(surface)

# Choose a layout for the graph
layout = g.layout('auto')

# Set up the alpha values for source and destination
alpha_at_src = 1.0
alpha_at_dest = 0.2

# Initialize the AlphaVaryingEdgeDrawer
drawer = ig.drawing.AlphaVaryingEdgeDrawer(ctx, alpha_at_src=alpha_at_src, alpha_at_dest=alpha_at_dest)

# Draw the directed edges using varying alpha
for edge in g.es:
    src_vertex = layout[edge.source]
    dest_vertex = layout[edge.target]
    drawer.draw_directed_edge(edge, src_vertex, dest_vertex)

# Save the image to a file
surface.write_to_png('graph_with_varying_alpha_edges.png')
<|EOT|>
