Transparent pngs with Django and matplotlib - django

Question

I currently have a Django project, with a view function, with code like the following (code copied from this post):
from pylab import figure, axes, pie, title
from matplotlib.backends.backend_agg import FigureCanvasAgg
def test_matplotlib(request):
f = figure(1, figsize=(6,6))
ax = axes([0.1, 0.1, 0.8, 0.8])
labels = 'Frogs', 'Hogs', 'Dogs', 'Logs'
fracs = [15,30,45, 10]
explode=(0, 0.05, 0, 0)
pie(fracs, explode=explode, labels=labels, autopct='%1.1f%%', shadow=True)
title('Raining Hogs and Dogs', bbox={'facecolor':'0.8', 'pad':5})
canvas = FigureCanvasAgg(f)
response = HttpResponse(content_type='image/png')
canvas.print_png(response)
return response
It creates a png from a graph and serves it directly. How can I make the background of the graph transparent in this png?

Answer 1

Untested, but this should work. You don't have to write to an (actual) file, a file-like object should work. This code saves the image into the response.
def test_matplotlib(request):
f = figure(1, figsize=(6,6))
ax = axes([0.1, 0.1, 0.8, 0.8])
labels = 'Frogs', 'Hogs', 'Dogs', 'Logs'
fracs = [15,30,45, 10]
explode=(0, 0.05, 0, 0)
pie(fracs, explode=explode, labels=labels, autopct='%1.1f%%', shadow=True)
title('Raining Hogs and Dogs', bbox={'facecolor':'0.8', 'pad':5})
#canvas = FigureCanvasAgg(f)
response = HttpResponse(content_type='image/png')
f.savefig(response, transparent=True, format='png')
#canvas.print_png(response)
return response