Jos Vennik

As promised, I’m going to display a simple way to generate a piechart through a form in Django, using Pycha and Cairo. You’ll need to get the libraries for both naturally (Google is your friend). Do note that I’m showing it to you in the simplest of ways.

Starting off right away with the forms.py:

We’re creating a small form with 8 input values. 4 Labels for the corrosponding 4 values, as shown below.

class PieChartForm(forms.Form):
val_1_label = forms.CharField(max_length=45)
val_1 = forms.IntegerField()

val_2_label = forms.CharField(max_length=45)
val_2 = forms.IntegerField()

val_3_label = forms.CharField(max_length=45)
val_3 = forms.IntegerField()

val_4_label = forms.CharField(max_length=45)
val_4 = forms.IntegerField()

That is basically all we need as far as the forms go. Moving on to the views.py, we’ll start off with some essential imports.

import cairo
import pycha.pie
import pycha.bar
from pycha.chart import Option

Moving on to the function itself.

def piechart(request):
if request.method == ‘POST’:
form = PieChartForm(request.POST)
if form.is_valid():

This bit checks whether there is POST data availablem if so it requests the data for the form. Then it checks whether the form data is actually valid. Following is a lengthy-ish bit of code to sort out all the data.

graph_lable1 = request.POST.get(’val_1_label’).encode(’latin-1′)
graph_lable2 = request.POST.get(’val_2_label’).encode(’latin-1′)
graph_lable3 = request.POST.get(’val_3_label’).encode(’latin-1′)
graph_lable4 = request.POST.get(’val_4_label’).encode(’latin-1′)

graph_val1 = int(request.POST.get(’val_1′))
graph_val2 = int(request.POST.get(’val_2′))
graph_val3 = int(request.POST.get(’val_3′))
graph_val4 = int(request.POST.get(’val_4′))

The above section places the post data into variables and casts them into strings and integers. As the generator cannot process unicode results. We move on by defining the actual canvas to draw on, including the insertion of data.

width, height = (500, 400)
surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, width, height)
dataSet = (
(graph_lable1, ((0, graph_val1), (0, 0))),
(graph_lable2, ((0, graph_val2), (0, 0))),
(graph_lable3, ((0, graph_val3), (0, 0))),
(graph_lable4, ((0, graph_val4), (0, 0))),
)

In this case it’ll be a 500 x 400 canvas. Also, the additional zeroes are indeed required if you wish for the generator to actually generate anything other than an error. Next up we can set some options for the generator.

options = {
‘legend’: {’hide’: False},
‘background’: {’color’: ‘#fff’},
‘colorScheme’: ‘blue’,
}

The options within do exactly what they say, so I won’t be going into that. We continue by actually saving the PNG file.

filename = ‘/home/user/png-gen/png-file.png’
chart = pycha.pie.PieChart(surface, options)
chart.addDataset(dataSet)
chart.render()
surface.write_to_png(filename)

Naturally, you can do a lot of things with the filename. I created the function for a website that allows the creation of this within projectgroups. So we needed a way to allow for several files to be generated for a group instead of having them overwrite eachother continuously. This is the variable I used:

current_time = str(datetime.datetime.now().strftime(’%Y-%m-%d-%H:%M:%S’))
filename = settings.MEDIA_ROOT + ‘/project_files/’ + str(project_id) + ‘/graphs/piechart-’ + current_time + ‘.png’

If you’ve completed all the above steps, you’ll need to return the function to something. You could use a render_to_response to write the generated image back to a template. Like so:

return render_to_response(”app/media_render.html”, locals(), context_instance = RequestContext(request))

You can add the following for the form validation and for when there is no POST data available:

else:
# If the form does not return valid data, provide an alert message and redirect link to the previous page
return HttpResponse(”Error message here”)
else:
# If there is no POST data, generate the form
form = PieChartForm()
return render_to_response(”businessplan/piechart.html”, {’form’: form}, context_instance = RequestContext(request))

Good luck!

P.S. Do mind the indenting, that’s something WordPress does not do so well without the code plugin that I haven’t gotten around to installing just yet.