**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
"""Shows all tries."""
        default_dag_run = conf.getint('webserver', 'default_dag_run_display_number')
        dag_id = request.args.get('dag_id')
        dag = current_app.dag_bag.get_dag(dag_id)
        base_date = request.args.get('base_date')
        num_runs = request.args.get('num_runs')
        num_runs = int(num_runs) if num_runs else default_dag_run
        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, y_axis_format='d', height=chart_height,
            width="1200")
        for task in dag.tasks:
            y_points = []