title: Setting app:layout_constraintTop_toBottomOf on ConstraintLayout
body: <p>I thought only by using <code>app:layout_constraintTop_toBottomOf="@+id/label_proximity"</code> is enough to set the <code>ImageView</code> below <code>TextView</code>. Do I need to  use <code>marginTop</code> to have both <code>TextView</code> visible? Or am I missing any attribute? Why in this case constraint is not working?</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;android.support.constraint.ConstraintLayout 
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:layout_margin="16dp"
  tools:context=".MainActivity"&gt;
  
  &lt;TextView
      android:id="@+id/label_light"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="@string/label_light"
      app:layout_constraintLeft_toLeftOf="parent"
      app:layout_constraintTop_toTopOf="parent" /&gt;
  
  &lt;TextView
      android:id="@+id/label_proximity"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="@string/label_proximity"
      app:layout_constraintLeft_toLeftOf="parent"
      app:layout_constraintTop_toBottomOf="@+id/label_light" /&gt;
  
  &lt;ImageView
      android:id="@+id/imageView"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:layout_marginTop="50dp"
      app:layout_constraintLeft_toLeftOf="parent"
      app:layout_constraintTop_toBottomOf="@+id/label_proximity" /&gt;
  
  &lt;/android.support.constraint.ConstraintLayout&gt;
  </code></pre>