**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
x_points = []
            for ti in task.get_task_instances(start_date=min_date, end_date=base_date):
                dttm = wwwutils.epoch(ti.execution_date)
                x_points.append(dttm)
                
                y_points.append(ti.prev_attempted_tries)
            if x_points:
                chart.add_serie(name=task.task_id, x=x_points, y=y_points)
        tis = dag.get_task_instances(start_date=min_date, end_date=base_date)
        tries = sorted(list({ti.try_number for ti in tis}))
        max_date = max([ti.execution_date for ti in tis]) if tries else None
        session.commit()
        form = DateTimeWithNumRunsForm(data={'base_date': max_date,
                                             'num_runs': num_runs})
        chart.buildcontent()
        return self.render_template(
            'airflow/chart.html',
            dag=dag,
            demo_mode=conf.getboolean('webserver', 'demo_mode'),
            root=root,
            form=form,
            chart=Markup(chart.htmlcontent),
            tab_title='Tries',
        )
    @expose('/landing_times')
    @has_dag_access(can_dag_read=True)
    @has_access
    @action_logging
    @provide_session
    def landing_times(self, session=None):