**Apache Airflow version**:

master

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**: breeze

**What happened**:

This DAG
```python
from airflow import models
from airflow.operators.dummy_operator import DummyOperator
from airflow.utils.dates import days_ago

with models.DAG("test", start_date=days_ago(1), schedule_interval=None,) as dag:
    t1 = DummyOperator(task_id="t1")
    t2 = DummyOperator(task_id="t2")
    t1 >> t2
```

is rendering like that:

<img width="1374" alt="Screenshot 2020-08-27 at 19 59 41" src="https://user-images.githubusercontent.com/9528307/91478403-11d7fb00-e8a0-11ea-91d0-d7d578bcb5a2.png">

**What you expected to happen**:

I expect to see same relations as defined in DAG file

**How to reproduce it**:

Render the example DAG from above.


**Anything else we need to know**:

I'm surprised by this bug 👀 

airflow/www/views.py
if dag is None:
            flash('DAG "{0}" seems to be missing.'.format(dag_id), "error")
            return redirect(url_for('Airflow.index'))
        if base_date:
            base_date = timezone.parse(base_date)
        else:
            base_date = dag.get_latest_execution_date() or timezone.utcnow()
        dates = dag.date_range(base_date, num=-abs(num_runs))
        min_date = dates[0] if dates else timezone.utc_epoch()
        root = request.args.get('root')
        if root:
            dag = dag.sub_dag(
                task_regex=root,
                include_upstream=True,
                include_downstream=False)
        chart_height = wwwutils.get_chart_height(dag)
        chart = nvd3.lineChart(
            name="lineChart", x_is_date=True, height=chart_height, width="1200")
        cum_chart = nvd3.lineChart(
            name="cumLineChart", x_is_date=True, height=chart_height, width="1200")
        y_points = defaultdict(list)
        x_points = defaultdict(list)
        cumulative_y = defaultdict(list)
        task_instances = dag.get_task_instances(start_date=min_date, end_date=base_date)
        ti_fails = (
            session.query(TaskFail)
            .filter(TaskFail.dag_id == dag.dag_id,
                    TaskFail.execution_date >= min_date,
                    TaskFail.execution_date <= base_date,
                    TaskFail.task_id.in_([t.task_id for t in dag.tasks]))