Trend

A counter or gauge shows one number. A trend adds how it moved: a small line, and the change of the last point against the one before it.

Give the metric a trend function that returns a time series, the same rows a time series insight returns:

from django.contrib.auth import get_user_model
from django.db.models import Count
from django.db.models.functions import TruncMonth

from django_insights.metrics import metrics

User = get_user_model()


def signups_per_month():
    return (
        User.objects.annotate(month=TruncMonth("date_joined"))
        .values("month")
        .annotate(total=Count("pk"))
        .values_list("month", "total")
        .order_by("month")
    )


@metrics.counter(question="How many users are there?", trend=signups_per_month)
def count_users() -> int:
    return User.objects.count()

It works the same for a gauge.

What the trend describes #

The trend is what its query returns, not the value itself. The example shows the signups per month next to the total number of users, so the line and the number answer two related questions. For a gauge such as a refund rate, return the rate per month, see Recipes.

• The trend is read from your own data on every collect and stored with the metric. It is rebuilt together with everything else, so nothing needs a backup.
• The latest 60 points are kept.
• Rows without a value are skipped, and fewer than two points show no line.
• Leave out the running month: it is not complete yet and shows as a drop. Filter it away in the query, for example with created_at__lt=start_of_this_month.