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Various programming stuff

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Using matplotlib to generate graphs in Django

Nowadays the most common way to generate graphs in your Django apps (or web apps in general) is to pass the data as json to the page and use a javascript lib. The big advantage these javascript libs offer is interactivity: You can hover over points to see their values making studying the graph much easier.

Yet, there are times where you need some simple (or not so simple) graphs and don’t care about offering interactivity through javascript nor you want to mess with javascript at all. For these cases you can generate the graphs server-side using django and the matplotlib plot library.

matplotlib is a very popular library in the scientific cycles. It can be used to create more or less any kind of graph and has unlimited capabilities! I won’t go into much detail about matplotlib here because the subject is huge but I recommend you to take a look at the comprehensive tutorials on its homepage.

To install matplotlib on unix you need to do a pip install matplotlib while, for windows, you can download the proper ready-made binaries from the Unofficial Windows Binaries for Python Extension Packages site that offers pre-compiled versions of almost all python packages! Just make sure to download the correct version for your python version and architecture (32bit or 64bit). After you’ve downloaded the file you can install it for your project using something like pip install matplotlib-3.3.4-cp38-cp38-win32 from inside your virtual environment.

Before actually creating a graph I recommend playing a bit with matplotlib to understand the basic concepts. Start a django shell and do the following:

>>> import matplotlib.pyplot as plt
>>> fig, ax = plt.subplots()
>>> ax.plot([1, 2, 3, 4], [1, 4, 2, 3])
[<matplotlib.lines.Line2D object at 0x0FBF5F58>]
>>> fig.show()

The above should open a window and display the graph. This works fine on Window 10 with python 3.8 and matplotlib 3.3.4 but I can’t guarantee other versions. If however fig.show() shows an error or does not display the graph, you can just do something like:

>>> fig.savefig('test')

that will output the figure in a file named test.png which you can the view. Please notice that the above are with the default options; there are various ways that matplotlib can be configured.

In any case, after you’ve played a bit with the shell and generate a nice figure (take a look at the matplotlib examples for inspiration) you are ready to integrate matplotlib with Django!

I can think of two ways which you can integrate matplotlib with Django:

  • Use a special view that would render the graph and just return a PNG object. Use a normal <img> element pointing to that view in your template.
  • Put the graph in the context of a normal django view encoded as a base64 object and use a special <img> with an src attribute of data:image/png;base64,{{ graph }} to actually embed the image in the template!

I prefer the second approach because it’s much more flexible since you don’t need to create a different Django view for each graph you want to generate. For this reason I will explain this approach right now and give you some hints if you need to follow the dedicated graph view approach.

Our view should:

  • Generate the graph
  • Save it in a BytesIO object
  • Convert that BytesIO to base64
  • Put the string value of the base64 encoded graph to the template

Then the template will just output that base64 value using the special img we mentioned above.

Here’s a snippet of a view that does exactly this:

import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt
import matplotlib.dates as mdates
import io, base64
from django.db.models.functions import TruncDay
from matplotlib.ticker import LinearLocator

class SampleListView(ListView):
  model = Sample

  def get_context_data(self, **kwargs):

    by_days = get_queryset().annotate(day=TruncDay('created_on')).values('day').annotate(c=Count('id')).order_by('day')
    days = [x['day'] for x in by_days]
    counts = [x['c'] for x in by_days]

    fig, ax = plt.subplots(figsize=(10,4))
    ax.plot(days, counts, '--bo')

    fig.autofmt_xdate()
    ax.fmt_xdata = mdates.DateFormatter('%Y-%m-%d')
    ax.set_title('By date')
    ax.set_ylabel("Count")
    ax.set_xlabel("Date")
    ax.grid(linestyle="--", linewidth=0.5, color='.25', zorder=-10)
    ax.yaxis.set_minor_locator(LinearLocator(25))

    flike = io.BytesIO()
    fig.savefig(flike)
    b64 = base64.b64encode(flike.getvalue()).decode()
    context['chart'] = b64
    return context

Please notice that after importing matplotlib I’m using the matplotlib.use('Agg') command to use the Agg backend. You can learn more about backends here, but it should be sufficient for now to know that using the Agg you’ll be able to save your graphs in png.

The above code uses some Django ORM trickery to group values by their created_on day value and then assings the days and counts to two arrays (days, counts). It then creates a new empty graph with a specific size using fig, ax = plt.subplots(figsize=(10,4)) and plots the data with some fancy styles with ax.plot(days, counts, '--bo'). After that it sets various options in the graph like the labels, grid etc.

The save and convert to base64 part follows: A new file like object is created using io.BytesIO() and the figure is saved there (fig.savefig(flike)). Then it is converted to a base64 string using the b64 = base64.b64encode(flike.getvalue()).decode(). Finally it is just passed to the context of the template as chart.

Now, inside the template I’ve got the following line:

<img src='data:image/png;base64,{{ chart }}'>

This will include the data of the chart inline and display it as a png image. If you’ve followed along you should be able to see the graph when you load that view!

If instead of including the graphs in your normal django template views you want to use a dedicated graph-generating view, you can follow my Django non-HTML responses tutorial. You could then modify the render_to_response method of your view like this:

def render_to_response(self, generator, **response_kwargs):
    response = HttpResponse(content_type='image/png')

    fig, ax = plt.subplots(figsize=(10,4))
    # fill the report here

    fig.savefig(response)
    return response

Since response is a file-like object you can save your graph directly there!

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