<｜begin▁of▁sentence｜>### Instruction:
As a personal trainer, you may often work with clients who are visual learners and benefit from seeing their progress graphically. Using networkx and the DarkToLightEdgeDrawer class from the igraph API, you can visually represent a client's workout progress over time. This graph will display directed edges with a gradient from dark (representing high intensity at the source) to light (representing low intensity at the destination). Additionally, you can save this visualization to a PNG file, allowing the client to refer back to it as a visual record of their fitness journey. This approach can help motivate clients, track performance, and guide adjustments to their training programs.
### Response:
import igraph as ig
import cairo

g = ig.read('graph19.gml')

surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, 400, 400)
ctx = cairo.Context(surface)

palette = ig.GradientPalette('black', 'red')

edge_drawer = ig.drawing.DarkToLightEdgeDrawer(ctx, palette)

plot = ig.Plot(surface, bbox=(400, 400), background='white')
plot.add(g)
plot.edge_drawer = edge_drawer
plot.redraw()

surface.write_to_png('graph_with_dark_to_light_edges.png')
<|EOT|>
