<｜begin▁of▁sentence｜>### Instruction:
In the realm of microbiology research, imagine you're exploring the intricate interactions between different microbial species within a complex ecosystem. You've collected data representing these interactions, akin to a directed graph indicating how various microbes influence one another.

Now, your task is to visually represent these interactions using a specialized tool called LightToDarkEdgeDrawer provided by the igraph API. This tool enables you to create a visual representation where edges transition from light (transparent) at the source microbe to dark (opaque) at the destination microbe, reflecting the intensity or significance of the interaction.

Once you've crafted a custom draw function utilizing the LightToDarkEdgeDrawer, you'll apply it to your microbial interaction graph. This will generate a visually compelling representation that highlights the dynamics of microbial influence within the ecosystem.

To ensure your findings are effectively communicated, you'll save this visualization as an image file named 'graph_light_to_dark.png'. This image will serve as a valuable reference for further analysis and discussion within the microbiology research community.
### Response:
import igraph as ig
import cairo

# Function to draw a graph
def draw_graph(graph, filename):
    visual_style = {}
    # Set bbox and margin
    visual_style['bbox'] = (300, 300)
    visual_style['margin'] = 20

    # Create a Cairo surface to draw on
    surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, 300, 300)
    context = cairo.Context(surface)

    # Instantiate our new edge drawer
    edge_drawer = ig.LightToDarkEdgeDrawer(context)

    # Plot the graph
    plot = ig.Plot(surface, **visual_style)
    plot.add(graph)

    # Redefine the edge drawer
    plot.redraw()
    for edge in graph.es:
        edge_drawer.draw_directed_edge(graph, edge, 1.0)

    plot.save(filename)

# Create a graph
graph = ig.Graph([(0, 1), (1, 2), (2, 3)], directed=True)
# Draw the graph
filename = 'graph_light_to_dark.png'
draw_graph(graph, filename)
<|EOT|>
